By A, M. PATERSON, M.D., Senior Demonstrator of Anatomy, and Lecturer on Dental Anatomy and Physiology in the Owens College, Manchester

Home , Intercostal nerves, Lumbar nerves, Sacral plexus

THE LIMB PLEXUSES OF MAMMALS.1 By A, M. PATERSON, M.D., Senior Demonstrator of Anatomy, and Lecturer on Dental Anatomy and Physiology in the Owens College, Manchester. (PLATE XIV.)

INTRODUCTION. THIS research was originally undertaken with the object of discovering, if possible, the significance of the spinal nerve plexusess. The primary aim was to find out why and how the spinal nerves came to be joined together to form nerves of distribution. Why do the plexuses exist? Why does not any given nerve spring from a single primary division instead of from several? In other words, why does not one primary division contain within itself all the strands of the nerve? How is it that a nerve does not pass directly to the parts which it supplies; but, on the contrary, gives fibres to, and gets fibres from, adjacent nerves in its course ? These plexuses are constant in their presence in the higher vertebrates, but no satisfactory reason for their existence is assigned by either the anatomist or the physiologist. Confining my attention in the first place to the morphology of the 'spinal nerves in mammals, I have made -complete dissections of ten different animals, with the object of discovering if there is any fundamental plan upon which the formation of the plexuses and the distribution of the nerves is based. These dissections have led to certain conclusions regarding the constitution of the plexuses, which are supported by a reference to the condition of things which obtains in man, and still further to the formulation of a general hypothesis which is adequate to explain the existence of these plexuses in relation to the limbs. In the present paper I propose to take up the subject in the following order: 1 The following memoir embodies some of the results of an investigation on the spinal nervous system of the Mammalia, which was presented to the University of Edinburgh as a Graduation Thesis for the degree of Doctor of Medicine in 1886 for which a Gold Medal was awarded by the Faculty of Medicine. 612 DR A. M. PATERSON. Firstly, To describe the constitution of the limb plexuses in a typical mammal. Secondly, To compare together the limb plexuses in different animals. Thirdly, To state the general conclusions derived from this comparison.

I. DESCRIPTION OF THE LIMB PLEXUSES IN THE PORCUPINE (Atherura fasciculata). The porcupine has been selected for this description, as it affords a typical example of the constitution of the plexuses, and the arrangement of the nerves to the limbs. In the following account the nomenclature adopted is as follows:-The two primary divisions of the nerve, resulting from the division of the original mixed nerve, are called the superior and inferior primary divisions. When these latter subdivide, the chief trunks are called dorsal and ventral.

A. Brachial Plexus (Plate XIV. fig. 1). This plexus is formed by the inferior primary divisions of the last three cervical, and the greater part of the first thoracic, nerves. The nerves emerge between the scaleni muscles, the plexus formation occurring in the axilla. The sixth cervical nerve (6th), on its appearance, gives off, firstly, -a branch which joins the phrenic nerve (Phr.),-formed by the fifth and sixth cervical nerves. From the branch thus given of, before its junction with the phrenic, comes the small nerve to the subelavius (Subcl.). The phrenic nerve courses backwards over the roots of the posterior cervical nerves into the chest. The inferior division of the sixth nerve then divides into one ventral and three dorsal branches. Of the dorsal, the first is the suprascapular (Ss); the second, a small nerve, is the first short subscapular (1 sub.); and the third is the first root of the circumflex nerve (Ci). This joins the second root, from the dorsal division of the seventh nerve. The ventral branch of the nerve forms a loop (x) with part of the ventral division of the seventh nerve, and gives rise to the musculo-cutaneous (M. C.). These nerves arise in the order given from before backwards. The seventh nerve (7th) splits into dorsal and ventral parts. The ventral part subdivides into anterior and posterior branches. The THE LIMB PLEXUSES OF MAMMALS. 613 anterior branch forms a loop (x) with the ventral part (v) of the sixth nerve. Before doing so, it gives off two fine nerves (P. M.) to the pectoralis major muscle. From the loop (x) come, at the one end the musculo-cutaneous nerve (M. C.), continuing outwards the direction of the nerve from the sixth; at the other end, the external anterior thoracic nerve (E. A. .T.), continuing outwards the branch from the seventh nerve. The latter nerve supplies the pectoralis major, and, piercing the muscle, gives small branches to the skin over it. The posterior branch of the ventral division of the seventh nerve forms the first root of the median (M.). The dorsal division also splits into anterior and posterior branches. The anterior branch forms the second root of the circumflex (Ci), joining part ofthe dorsal division of the sixth nerve. The posterior branch forms the first root of the musculo-spiral (A. S.). From each of these branches a nerve is given off before they join their companion nerves. From the anterior branch a special subscapular nerve to the teres major (T. M.) arises, before the formation of the circumflex. From the posterior branch the long subscapular nerve is given off (L. sub.) to the latissimus dorsi. The circumflex nerve in the axilla gives origin to two small nerves, the middle subscapular (M. sub.) supplying part of the subscapularis and the teres major; and the second short subscapular nerve (2 sub.) to the muscle of the same name. lThe eighth nerve (8th) divides into ventral and dorsal portions. The ventral and larger portion is in two parts, one passing outwards to form the second head of the median (M.), the other passing backwards to join the ventral division of the first thoracic nerve, and being the first root of the " lateral cutaneous nerve of the thorax 2 1 (L. C. T.). The dorsal smaller portion forms the second (and hprger) root of the musculo-spiral (A. S.). This nerve is thus derived from the dorsal divisions of the seventh and eighth nerves. The first thoracic nerve (T. 1st) divides at its emergence into five branches, as follows: 1. A large branch which passes backwards a short distance, and is joined by part of the ventral division of the eighth nerve to form the "lateral cutaneous nerve of the thorax" (L. C. T.). This nerve, soon subdivides into three large branches, which are distributed over the chest-wall. One (a) enters the panniculus carnosus and supplies it and the skin of the posterior part of the body wall. The other two branches (b) pass backwards and under cover of a large muscle, which extends forwards from the abdomen to the sternum, its fibres directed forwards and downwards; beneath this muscle these two nerves can be traced into the abdominal wall. 1 This nerve, which is fairly constant as a considerable trunk in mammals,- being found in seven out of the ten animals dissected, -may be looked upon as the homologue of the nerve of Wrisberg (lesser internal cutaneous) of human anatomy. Its origin is the same (from the ventral trunks of the eighth cervical and first thoracic nerves) ; and it supplies the integument of the fold of the axilla, communicating in its course with the intercostal nerves. In these animals, moreover, there is no other nerve corresponding to the nerve of Wrisberg. 614 DR A. M. PATERSON. 2. The internal anterior thoracic (I. A. T.). This nerve passes above the first root of the last named nerve, and supplies the pectoralis minor. It is but small. 3. The third head of the median (M.). This nerve, formed from the ventral divisions of the seventh and eighth cervical nerves, and from the first thoracic, is the largest in the plexus. 4. The ulnar nerve (U.). Smaller than the median, and arising behind it. 5. The internal cutaneous nerve (I. C.). Small and arising behind the ulnar. The three last nerves, median, ulnar, and internal cutaneous, pass together to the fore-limb. The remaining smaller portion of the first thoracic nerve con- tinues downwards through the first intercostal space, supplying the muscles, but giving no cutaneous branches. It is to be remarked, regarding the part which this nerve plays in the formation of the brachial plexus, that it gives no trunk corresponding to the dorsal trunks of the other nerves engaged. In this respect it is an exception. In eight out of the other nine animals dissected the axillary portion of the first thoracic nerve resembled the other nerves joining the plexus, and, like them, divided into dorsal and ventral parts, which combined with the corresponding cords of anterior nerves to complete the plexus. The other exception was the rat, in which animal the nerve agreed with the one now under consideration in giving off no dorsal branch. Distribution of the Nerves of the Brachial Plexus. The nerve to the serratus magnus muscle is formed by two roots, one from the seventh, one from the eighth cervical nerve, in the substance of the scalene muscle. It emerges at the superior border of the serratus, and enters the substance of the muscle. Beyond this there is little to add. The musculo-cutaneous and internal cutaneous nerves have their usual distribution. The median supplies the muscles of the fore-arm and hand, and the skin of 31 digits in the palmar aspect radial. It gives off an anterior interosseous branch. The ulnar has its usual course. It gives small muscular nerves in the front of the fore-arm; a dorsal branch to lJ fingers, which passes beneath the tendon of the flexor carpi ulnaris; it ends in the palm by supplying 1 digits on the ulnar side, and by giving branches to the muscles. The polled is diminutive. The musculo-spiral supplies the extensor muscles of the elbow and hand, and gives off radial, external cutaneous, and posterior interosseous branches. The radial appears from beneath the tendon of the supinator longus muscle, and supplies 3J digits dorsally on the radial side.

B. Lwmbo-sacral Plexus (figs. 2 and 3). For the sake of convenience this plexus will be described as THE LIMB PLEXUSES OF MAMMALS. 615 consisting of two parts, lumbar and sacral, although here, as in other cases, there is no line of demarcation between them, and certain nerves enter into the formation of both. The lumbar plexuds (fig. 2) is formed by the first four lumbar nerves. 7Wefirst nerve (1st) gives off at its root a small branch, which passes backwards and forms a loop with the second. On its way this branch gives off two nerves. 1. The genito-crural (G. C.); this pierces the psoas magnus (Ps. m.), and appears on its abdominal surface internal to the psoas parvus (Ps. p.). Passing backwards over the psoas to the thigh, it ends by dividing into internal (genital) and external (crural) branches. 2. The first root of the external cutaneous nerve (E. C.). The main trunk of the first nerve -passes through, and appears at the outer border of, the psoas magnus, as the ilio-hypogastric (1 H.). This gives off a large lateral (iliac) branch (L.) which pierces the abdominal muscles, and supplies the flank. The inferior (hypogastric) branch passes downwards between the muscles, and ends in the hinder part of the abdominal wall, by dividing into several fine branches after passing in front of the " anterior superior spine " of the ilium. The second nerve (2nd) is joined on its emergence by a loop from the first. It gives the following branches: 1. Two muscular nerves (x. x.) to the psoas. 2. The second root of the external cutaneous (E. C.). The two roots unite in the psoas and appear on its surface as one nerve external to the tendon of the psoas parvus, and posterior to the genito-crural. This nerve courses outwards over the iliacus, and, passing to the thigh under Poupart's ligament, ends in cutaneous branches. 3. The main portion of the nerve passes backwards close to the root of the transverse process, and opposite to the point of emergence of the third nerve divides into ventral (small) and dorsal (larger) branches. The ventral branch forms the first root of the obturator (Obt.), the dorsal branch, the first root of the anterior crural nerve (A.C.) [lThe third nerve (3rd) divides at its origin into ventral and dorsal parts. The ventral (smaller) part is the second (and largest) root of the obturator nerve (Obt.). The dorsal (larger) part is the second root of the anterior crural (A. C.). It is also the largest root. They join respectively the ventral and dorsal parts of the second nerve, and the cords formed pass backwards in the substance of the psoas. The fourth nerve (4th) gives off at its origin a small dorsal branch, the third root of the anterior crural (A. C). This nerve, derived from dorsal divisions of the second, third, and fourth nerves, pierces the psoas magnus, dividing its tendon into two parts, appears in the 616 DR A. M. PATERSON. iliac fossa, and passes into the thigh. In the false pelvis it supplies muscular branches to the iliacus. The main trunk passes backwards to join the fifth. On its way a small ventral branch arises-the third root of the obturator (Obt.). This nerve, formed from ventral branches of the second, third, and fourth nerves, appears at the inner border of the psoas magnus; enters the pelvis, and leaves it by the thyroid foramen. The rest of the fourth nerve, as remarked, joins the fifth; their union gives rise to the lumbar cord, which enters into the formation of the sacral plexus. Distribution of the Nerves of the Lumbar Plexus. In addition to what has been said incidentally above, the following may be stated :-The external cutaneous supplied the front and outer side of the thigh. The anterior crural supplied the muscles on the front of the thigh, and gave off the internal cutaneous and internal saphenous branches. The latter was traced to the foot, where it supplied the inner side and great toe, and was distributed on the dorsum of the foot to the clefts between the three inner toes. The obturator supplied the muscles on the inner side of the thigh. The sacral plexus (fig. 3) is formed by part of the fourth, the fifth lumbar, and the first two sacral nerves. The fifth lumbar nerve (5th) gives off firstly a small branch (Py.) which supplies the pyriformis muscle on its pelvic aspect. The trunk of the nerve then unites with the greater part of the fourth to form a stout cord, which soon divides into ventral and dorsal portions. Before the junction of the trunks, each gives off a slender branch, by the union of which is formed the superior gluteal nerve; this passes out of the pelvis above the pyriformis, and supplies the usual muscles (S. G.). There is a trace of division in these cords from the fourth and fifth lumbar nerves, before their union, into ventral and dorsal parts; the ventral and dorsal trunks are united with corresponding trunks, to form the nerve, which, as above stated, soon shows a distinct division into ventral and dorsal portions. The dorsal portion forms the external popliteal nerve (E. P.). This, in the upper part of the thigh, gives off on the outer side two large nerves (h and A') to the gluteus muscle, one arising close to the pelvis. The ventral portion passes backwards, and is joined by a smaller trunk from the first sacral nerve, to form the internal popliteal (I. P.). The first sacral nerve (1 S.) gives off first a small branch, which passes out and joins the ventral trunk derived from the 4th and 5th lumbar nerves. The trunk resulting is the internal popliteal (I. P.) It soon subdivides into ventral (smaller) and dorsal (larger) parts. The former is a nerve to the hamstrings (H.); the latter is the internal popliteal nerve proper. From the nerve to the hamstring muscles (H.) come two branches to the gemelli (a.), one in conjunc- tion with a branch to one of the hamstrings. The internal popliteal nerve near its origin also gives a branch to a gemellus muscle. THE LIMB PLEXUSES OF MAMMALS. 617 These three nerves, external popliteal, internal popliteal, -and nerve to the hamstrings, then proceed down the back of the thigh, lying in that order from without inwards. The main part of the first sacral nerve passes backwards to join the second, giving off oil the outer side the inferior pudendal nerve (S.). This nerve, about half the size of the root which joins the internal popliteal, supplies one branch to the skin round the anus, and another which passes forwards towards the genital organs, and more externally. The first sacral nerve before joining the second divides into ventral and dorsal parts; each joins a corresponding part of the second to form by the ventral portions the pudic nerve (Pud.) by the dorsal portions, the small sciatic (S. Sc.). The second nerve (S. 2) gives off firstly a branch, which passes backwards to join the third, and then divides into two main portions- anterior (larger) and posterior (smaller). The larger part subdivides again into ventral and dorsal branches, which, by uniting with the ventral and dorsal branches respectively from the first, form the pudic (Pud) and small sciatic (S. Sc.) nerves. The pudic nerve has its usual distribution; the small sciatic supplies the skin of the upper part of the back of the thigh. The smaller (posterior) portion of the second nerve supplies by several branches the parts about the anus and root of the tail (x). The third and fourth sacral and the caudal nerves (fig. 3) are princi- pally concerned in the formation of the inferior caudal trunk, and may be left undescribed, as they are not concerned in the supply of the hind limb. Distribution of the Nerves of the Sacral Plexs. The external popliteal nerve has its usual course down the back of the thigh, below the head of the fibula, to the leg. It supplies cutaneous offsets in the ham and outer side of the leg, and the following terminal branches: recurrent tibiaJ, to the front of the knee- joint; anterior tibial and musculo-cutaneous. The anterior tibial nerve passes down the front of the leg, and ends on the dorsum. of the foot, by supplying the adjacent sides of the third and fourth toes. The musculo-cutaneous nerve supplies the muscles on the outer side of the leg, and ends in the dorsum of the foot, by supplying the adjacent sides of the fourth and fifth toes. The internal popliteal nerve gives off in the thigh a large external saphenous nerve, which passes down the leg and round the external ankle to the foot, the outer side of which it supplies, sending branches to the dorsum. In the ham the internal popliteal gives off many muscular and articular branches. It ends by passing into the sole behind the inner ankle.

II. ANALYSIS OF THE CONSTITUTION OF THE MAMMAILIAN LIMB PLEXUS. Besides the porcupine, the following nine animals have been 618 DR A. M. PATERSON. dissected :-the koala, rat, rabbit, guinea-pig, cat, camel, brindled gnu, capucinus and entellus monkeys. The distribution of the spinal nerves and the constitution of the limb plexuses in these animals have been described and figured in the memoir mentioned above. These dissections, along with published accounts of the spinal nerves in other mammals, give sufficient data to allow a general comparison to be made regarding the constitution of the plexuses. By an analysis of the different plexuses, both of the fore-limb and hind-limb in different animals, one finds that there are certain minor differences in the arrangement of the nerves in different cases; while, on the other hand, there are certain fundamental points in which all agree. A. Points of Difference. 1. The Nuwmber of Nerves entering each Plexuw.-Examining in detail the number of the nerves entering into the composition of the brachial and lumbo-sacral plexuses in the animals dissected, it is found that five nerves, or thereabouts, are engaged in forming the nerves of distribution to the limb proper. The number, however, varies slightly in different animals, being sometimes more and sometimes less than five. 2. Brachial Plexus.-The-subjoined table shows that in the animals dissected five nerves gave rise to this plexus in five instances. In four cases, four nerves were implicated, and in one (entellus monkey) six nerves entered into the plexus: Table A.

Cervical. Thoracic.

Rat, . . 5th, 6th, 7th, 8th. 1st (part), Guinea pig, . . 5th, 6th, 7th, 8th. 1st Rabbit, . . . 5th (part), 6th, 7th, 8th. 1st Koala, . . . 5th, 6th, 7th, 8th. 1st Capuchin monkey, 5th, 6th, 7th, 8th. 1st Entellus monkey, . 4th (part), 5th, 6th, 7th, 1st 8th. Porcupine, 6th, 7th, 8th. 1st Cat, . 6th, 7th, 8th. 1st Camel, . . . 6th (part), 7th, 8th. 1st Gnu, . . . 6th (part), 7th, 8th. 1st THE LIMB PLEXUSES OF MAMMALS. 619

The nerves constantly present are the sixth (or a part of it), seventh, and eighth cervical, and the first thoracic nerve. The nerves not constant in their presence are the fourth (present in the entellus monkey), and the fifth (present in five cases).

B. Lumbo-Sacral Plexus.'

In all cases but one the inferior, forming divisions of five spinal nerves are engaged in forming the plexus. In that one case (the porcupine) there are six nerves present. The following table shows the nerves engaged:- Table B.

Lumbar. Sacral. Rat, . . . 2nd, 3rd, 4th, 5th, 6th. Guinea pig, 3rd, 4th, 5th, 6th. 1st. Rabbit, . . 4th, 5th, 6th, 7th. 1st (part). Entellus monkey, 3rd, 4th, 5th, 6th, 7th ... (part). Cat, . 4th (part), 5th, 6th, 7th. 1st (part). kFive nerves. Koala, . . . 4th (part), 5th, 6th, 7th, ... 8th. Capuchin monkey, 2nd, 3rd, 4th, 5th. 1st (part). Camel, . ' 4th (part), 5th, 6th, 7th. 1st. Gnu, . . . 4th, 5th, 6th. 1st, 2nd. Porcupine, . . 1st (part), 2nd, 3rd, 4th, 1st (part). Six nerves. 5th.

In other recorded cases the same irregularity occurs in the number of nerves entering the plexuses. The number of component nerves varies from four to seven. In the brachial plexus the number may be four, as in the fox2 and the ass 2; five, as in the ox,3 sheep,3 pig,3 horse,4 carnivora,3 chimpanzee,5 cynocephalus anubis,5 thylacine,6 and cuscus 6; or six, as in man, the

1 In this section those nerves only are considered, as being derived from the lumbo-sacral plexus, which are distributed to the hind limb. The nerves corresponding to the iio-hypogastric and illo-inguinal, small sciatic, and internal pudic are therefore omitted. 2 Swan, Comp. Anat. of the Nervous System. 3 Chauveau, Comp. Anat. ofDomesticated Animals. 4 Chauveau, Comp. Anat ofDomesticated Animals; Percival, Anat. ofthe Horse. 5 Champneys, Jour. ofAnat. and Phys., vol. vi. p. 211. 6 Cunningham, D. J., Jour. of Anat. and Phys., vol. xii. p. 427. 620 DR A. M. PATERSON. porpoise,' dolphin,' and Macaque monkey.2 In the lumbo-sacral plexus the number may be four, as in thylacine 3; five, as in the pig,4 fox,5 and ass 5; six, as in the ox,4 horse,6 chimpanzee,7 cynocephalus anubis,7 and CUSCUS3; or seven, as in man. The limb plexuses, therefore, present slight differences in individual cases in respect to the number of nerves which form them. At the same time, however, the number in all cases is about five. This fact is of importance in giving support to the late Professor Goodsir's hypothesis8 that the mammalian limb derives elements from five vertebral somatomes. 2. Position of the Limb Plexvtses in the Series of Spinal Nerves.-In this respect, also, the differences in the several plexuses are only slight. In the brachial plexus the nerves which form it are always the sixth, seventh, eighth, and ninth spinal nerves, with, in certain cases, the fifth, or more rarely the fourth. This is seen in the Table (A) given above (page 618). Occasionally in man 9 the tenth spinal nerve joins the plexus. In the lumbo-sacral plexus, while there is considerable unifor- mity in the number of component nerves, there is at first sight no regularity in their relations to the vertebral segments (Table B, page 619). The plexus may be formed wholly by lumbar nerves, as in the rat, or by the addition of sacral nerves as well. This discrepancy is due to the variations in the number of thoracico-lumbar vertebrae, and in the position of the sacro-iliac articulation. It disappears to a large extent when the numbers of plexus-forming nerves are counted in relation to the series of the spinal nerves. The subjoined table gives the numbers of spinal nerves, and from the numbers entering the plexus which is distributed to the hind limb, it is noted that they vary from the twenty-first to the twenty-ninth, while the twenty-fifth nerve is the only one which is present in all cases. 'Cunningham, D. J., Jour. of Anat. and Phys., vol. xi. p. 209. 2 Brooks, W. T., Jour. of Anat. and Phys., vol xvii. p. 329. 3 Cunningham, D. J., Jour. of Anat. and Phys., vol. xv. p. 265. 4 Chauveau, Comp. Anat. of Domesticated Animals. 5 Swan, Comp. Anat. of the Nervous System. 6 Chauveau, Comp. Anat. ofDomesticated Animals; Percival, Anat. of the Horse. 7 Champneys, Jour. of Anat. and Phys., vol. vi. p. 211. 8 Goodsir, Edin. New Phil. Jour., new series, vol. v., Jan. 1857 ; Anat. Hem., vol. ii. p. 201, 1868. 9 Cunningham, D. J., Jour. of Anat. and Phys., vol xi. p. 539. THE LIMB PLEXUSES OF MAMMALS. 621 Table C.

Naunber of Spinal Nerves. Animals. Spinal Nerves in Lumbo-Sacra! Plexus. Cervical. Thoracic. Lumbar. Sacral. Caudal. Total.

Rat, . . 8 11 6 2 3 30 21st to 25th incl. Guineapig, 8 13 6 2 1 30 24th,, 28th Rabbit, . 8 12 7 3 4 34 24th,, 28th Porcupine, 8 14 5 4 5 36 23rd,, 28th Entellus monkey, 8 12 7 3 4 34 23rd,, 27th Cat, . . 8 13 7 3 6 37 25th,, 29th Koala, . 8 11 8 2 1 30 23rd,, 27th Capucinus monkey, 8 13 5 4 5 35 23rd, 27th Camel, . 8 12 7 5 5 37 24th, 28th Brindled gnu,. . 8 13 6 4 6 37 25th,, 29th

B. Points of Agreement in the Composition of the Plexuses. Notwithstanding these minor differences in the number of the component nerves of the limb plexuses, and their relative position in the series, there are certain points in which not only the same plexuses in each animal, but the plexuses both of the fore-limb and hind-limb agree together, and from which further generalizations can be made. These points of agreement may be classed under two heads, and treated separately: (1) The arrangement of the nerves in the plexuses; (2) the distribution of the nerves in the limbs. 1. The Arrangement of the Nerves in the Plexuses.-From the dissections and a comparison of results, the three following deductions may be made regarding the formation of the limb plexuses:- 1. The inferior primary divisions of the nerves entering the plexuses divide into dorsal and ventral trunks. 2. The dorsal divisions of the nerves always combine with dorsal divisions, the ventral divisions with ventral divisions, to form the nerves of distribution. 3. The essential constitution of a nerve of distribution con- sequently never varies. A nerve arising from a combination of the dorsal divisions of certain nerves in one animal is never found in another animal to spring from VOL. XXI. (N.S. VOL. I.) 2 T 622 DR A. M. PATERSON. the ventral divisions of these or any other nerves. The same is the case with nerves derived from ventral divisions. These three deductions are supported by an examination of both brachial and lumbo-sacral plexuses. In the case of the fore-limb the nerves of distribution are derived from the inferior primary divisions of the hinder cervical and first thoracic nerves. The nerves entering the plexus divide into ventral and dorsal parts, the ventral divisions of the nerves combining to form one set, the dorsal divisions combining to form another set, of nerves of distribution. In the case of the hind-limb, in the same way, the nerves of distribution are derived from combinations of ventral and dorsal divisions (respectively) of the inferior primary trunks of the posterior lumbar and anterior sacral nerves. In neither case do ventral divisions ever combine with dorsal divisions of adjacent nerves. In neither case does a nerve of distribution, derived in one animal from ventral divisions, in another case spring from dorsal divisions, and vice ver84. C. Brachial Plexu8.

Origin. Nerves. Distribution.

Anterior thoracic (internal Pectoral muscles. ( and external), Muscles on front of humerus, Musculo-cutaneous, . skin on outer side and front Ventral of fore-arm. Divisions,Diiios Median, Muscles and skin on front of Ulnar, f fore-arm and hand. Internal Skin on inner side and front of cutaneous,cutaneous,. arm and fore-arm. Suprascapular, . . . Muscles on dorsum of scapula. Short subscapular (varying Ssbcapularis muscle. in number), . . I Deltoid and teres minor Circumflex, . . muscles: skin on back and Dorsal outerside ofshoulder and arm. Divisions, Middle subscapular, Subsca ular and teres major Muscles on back of humerus, Musculo-spiral, . . . muscles and skin on back of fore-arm and hand. Long subscapular, . Latissimus dorsi muscle. THE LIMB PLEXUSES OF MAMMALS. 623 D. Lumbo-Sacral Plexus.

Origin. Nerves. Distribution.

Genito-crural, . . . Groin. Muscles on inner side of Ventral Obturator, back of knee-joint. thigh; Internal popliteal (part of f Muscles and skin on back of leg Divisions,Divisis, great sciatic), . . and sole of foot. Nerve to hamstrings (part Hamstring muscles. of great sciatic), (Muscles and skin in front of Anterior crural, . thigh; skin of inner side of Dorsal leg and foot. Gluteus medius, and Divisions. Superior gluteal, . ,tensor fascia femoris.minimum; External popliteal (part of \ Muscles and skin of outer side great sciatic), . . f and front of leg and foot.

The two preceding tables give in a summarised form the origin and distribution of the nerves derived (C) from the brachial, and (D) from the lumbo-sacral plexus. On the one side the general distribution of the nerves is given. On the other side is stated the origin of the nerves according to the- above plan,-that is, whether from dorsal or ventral divisions of the primary nerves. In the second table the following nerves are not entered:- External cutaneous, internal pudic, and small sciatic. The relations of the two last to the groups of ventral and dorsal divisions of the nerves are difficult and uncertain. They are both intimately connected together at their origin, and the distribution of the small sciatic nerve to the limb proper is only slight. It appears as if the pudic nerve represented the ventral portions, while the small sciatic nerve corresponded to the united dorsal divisions of certain nerves. The external cutaneous nerve, from its origin and distribution, is probably homologous with certain dorsal divisions; but as this is somewhat uncertain, the nerve, which in the animals dissected supplied the skin of the flank, and no great part of the limb proper, is for the present left out of account. It will be noted also that the great sciatic nerve, both in the account of the lumbo-sacral plexus of the porcupine and in the foregoing table, has been divided into three parts,- external popliteal, internal popliteal, and nerve to the hamstring 624 DR A. M. PATERSON. muscles. In -all the animals dissected (except the koala)' these nerves were separate up to their origin from the plexus.2 The internal popliteal and external popliteal nerves, though lying side by side, and more or less closely bound together in a fibrous sheath, were still distinct nerves with different origins. The external popliteal arises from the dorsal divisions; the internal popliteal, of which the nerve to the hamstrings is to be regarded as a branch, from the ventral divisions of certain inferior primary trunks. This condition of the great sciatic nerve I have shown to occur in man.3 In birds also, the two popliteal nerves are known to be separate up to their origin from the plexus.4 The limited number of dissections made do not justify more particular conclusions than those generalised in the rules formulated above. At the same time, however, there is invariably a fixed and definite relation between the different nerves derived from the plexuses, regarding their position and order of origin. That order has been preserved in the above tables, after a separation of the nerves according to their origin from groups of dorsal or ventral divisions of the component nerves. It corresponds with the arrangement met with in human anatomy; in regard to the nerves of the fore-limb, with the account given of the human brachial plexus by Dr Herringham,5 in regard to the hind-limb, with the arrangement noted by myself.6 2. Distribution of the Nerves inb the Limbs.-Passing, in the next place, from a consideration of the origin of the nerves, and the composition of the plexuses, to an examination of the parts of the fore and hind limbs supplied by the nerves derived from 1 This animal had already been partially dissected and kept some time in salt, so that the tissues had become hard, making its dissection somewhat difficult. 2 Since the above observations were made I have had an opportunity of notic- ing the same condition of the great sciatic nerve, in dissection mades by my colleague Mr Robinson, of the tapir (Tapirus Americanus), the beaver (Castor Canadensis), and the paca (Ccelogenys Paca). 3 Jour. of Anat. and Phys., vol xxi., April 1887. 4 Macartney, article " Birds " in Rees' Cyclopedia. 5 Proc. Roy. Soc. Lond., No. 249, 1887. 6 Jour. of Anat., vol. xxi., April 1887. THE LIMB PLEXUSES OF MAMMALS. 625 them, it is found that a similar classification to the above may be made with regard to distribution. The mammalian limbs originate as buds, which spring in pairs from the front and back of the Wolffian ridge, a fold which extends along the lateral wall of the embryo. These buds are directed downwards and outwards in the first instance, with an inclination backwards in the fore-limb, forwards in the hind- limb. Each presents a dorsal, superior, or external surface; a pre-axial and a post-axial border. In the centre of this primitive limb a formation of cartilage occurs, at first in the shape of an unsegmented bar or rod, which afterwards develops into the bony framework and joints. Outside this cartilaginous framework muscular envelopes are formed, giving rise to a double dorsal and a double ventral layer, at first continuous with one another at the end of the bud, and at the root of the limb with the muscular investment of the trunk. Primitively, then, there are present in the limb a dorsal and a ventral surface, each presenting cutaneous and muscular strata, lying respectively above and below the cartilaginous framework of the limb. These muscular and cutaneous strata have to be supplied with nerves. The subsequent changes in the limbs, first of elongation and angulation, next of rotation, forwards in the hind-limb, backwards in the fore-limb, produce important effects, not only in the external configuration, but also in the internal conformation of the limbs. Along with these changes, the development of the muscular strata into complicated systems of muscles increases the difficulty of making out the relations which exist between the parts of the fetal and the adult limb. It is sufficiently clear, however, for the object at present in view that in the case of the fore and hind limb respectively the following parts in the adult mammal are identical with, and developed from, the dorsal and ventral surfaces of the embryonic limbs. In the Fore-L'imb.-The parts on the scapula, the extensor surface of the humerus (back of arm), the extensor surface of radius and ulna (back of fore-arm), and the dorsum of the manus, are developed from the dorsal surface of the primitive limb. The pectoral muscles, the flexor surface of the humerus (front 626 DR A. M. PATERSON. of the arm), the flexor surface of radius and ulna (front of fore- arm), and the palmar surface of the manus, are developed from the ventral surface of the primitive fore-limb. In the Hind-Limb.-The region of the buttock, the extensor surface (or front) of the thigh, the front of the leg, and the dorsum of the foot, are serially continuous with, and developed from, the primitive dorsal surface. The parts in the inner side and back of the thigh (including the adductor and hamstring muscles), the back of the leg, and the plantar aspect of the pes, represent the ventral surface of the primitive limb. Turning again to the tables ofnerves (C and D, p. 622, e.8.), we find that the parts of the fore and hind limbs, which are stated to be derived from the originally dorsal surface of the (embryonic) limb, are supplied by nerves of distribution, which are formed by a combination of dorsal divisions of the nerves forming the limb plexus. Those parts of the limbs which are derived from the primitive ventral surfaces are supplied in the same way by nerves of distribution derived from the combinations of the ventral divisions of the nerves entering into the plexuses, This holds good, without exception, throughout the indiscriminate series of animals examined. An example may be cited in the case of the obturator and anterior crural nerves. The obturator nerve, derived from ventral divisions of certain lumbar nerves, and associated at its origin with the anterior crural, supplies the adductor muscles and skin in the inner side of the thigh and the back of the knee-joint-parts which, in an embryonic condition, belong to the ventral surface of the limb. The anterior crural nerve, invariably springmg from the dorsal divisions of certain lumbar nerves, supplies the muscles and skin on the front of the thigh. These axe parts of the originally dorsal surface of the limb. 3. Deducton from the, foregoing (Considerations.-From a consideration-first, ofthe arrangement ofthe origin ofthe nerves and the constitution of the plexuses; and, second, of the parts of the limbs supplied by them, viewed in the light of the development of the mammalian limb, the following hypothesis suggests itself: That in a primitive condition of the limb, at an early period of development, the nerves have a simple arrangement, and are THE LIMB PLEXUSES OF MAMMALS. 627 distributed to the simple bud which springs from the body-wall in the following way:-The more pre-axial nerves supply the pre-axial portion of the limb; the more post-axial, the post-axial portion of the limb; while the inferior primary division of each nerve engaged in the formation of the limb plexus, or the supply of the limb, divides into a dorsal and a ventral branch for the supply of the dorsal and ventral surfaces respectively of the embryonic limb. The embryonic nerves become differentiated and complicated in their arrangement, parn pas8t with the development of the muscular system, and the changes which take place in the production of the adult condition. This hypothesis is supported by various independent observations. Schwalbel figures a schematic arrangement of the human brachial plexus, in which he shows the inferior primary divisions of the component nerves (except the first thoracic) dividing at their origins into dorsal, or posterior; and ventral, or anterior, parts. He, moreover, classifies the nerves of distri- buition to the arm, in accordance with that view, into a dorsal or posterior set, derived from the dorsal divisions, and supplying the extensor surface of the limb; and a ventral or anterior set, derived from the ventral divisions, and supplying the flexor surface. Dr James Ross, of Manchester, has long taught, from certain pathological considerations, which need not be detailed here, that the cutaneous distribution of the nerves of the human arm bears a certain relation to the mode of development of the limb. He has shown that the fifth and sixth spinal nerves supply the outer side of the upper arm, fore-arm, and hand; while the axillary fold, the inner side of the upper arm, and the inner side of the fore-arm and hand are supplied by the tenth, ninth, and eighth spinal nerves. From this arrangement he was led to for- mulate the hypothesis that the limb in its development pushed itself out, so that the highest nerves supplied the most proximal and pre-axial part of the limb (cutaneously), the lowest nerves the most proximal and post-axial part; while the intermediate nerves were distributed to the intermediate portion, and were drawn furthest to the distal extremity. Professor Humphry, in his observations on myology, shows that the vertebrate limb I Lehrbuch der Neurologie, Erlangen, 1881, p. 914. 628 DR A. M. PATERSON. contains within itself prolongations of certain muscular strata of the body-wall, besides intrinsic muscles which are proper to the limb itself. In addition, he points out that the more superficial strata, both dorsally and centrally, terminate (either in the fascia or by definite osseous attachments, or by blending with deeper layers) nearer to the root of the limb than the deeper strata; while these again, in like manner, fall short of the terminations of the intrinsic muscles. Dr Herringham, in his memoir on the "Human Brachial Plexus,"' fully bears out Professor Humphry's observations in myology and Dr Ross's deductions in pathology, at the same time supporting the views which I venture to put forward in the sequel regarding the formation and development of the plexuses. By tracing the different nerves and their branches through the plexus to their ultimate distribution, and from a consideration of the muscles and cutaneous areas which they supply, he has made the following generalisations:-L"1. Of two muscles, or of two parts of a muscle, that which is nearer the head end of the body tends to be supplied by the higher; that which is nearer the tail end by the lower nerve. 2. Of two muscles, that which is nearer the long axis of the body tends to be supplied by the higher; that which is nearer the periphery by the lower nerve. 3. Of two muscles, that which is nearer the surface tends to be supplied by the higher; that which is further from it by the lower nerve. 4. Of two spots on the skin, that which is nearer the pre-axial border tends to be supplied by the higher nerve. 5. Of two spots in the pre-axial area, the lower tends to be supplied by the lower nerve; and of two spots in the post-axial area, the lower tends to be supplied by the higher nerve." Positive proof of the truth of the foregoing hypothesis can be obtained from an examination of the successive stages of the development of the spinal nerves in the embryo. It would ex- tend the limits of this memoir to too great a length if the attempt were made to trace the development of the nerves up to the formation of the plexuses. For the present purpose it is sufficient to note that inthefowl the plexusesare arranged in the same general way as in mammals,2 and obey the same laws in the constitution of the nerves of origin and distribution respectively. 1 Op. cit. 2 Macartney, loc. cit. THE LIMB PLEXUSES OF MAMMALS. 629 Working with chick embryos, I have traced the development of the spinal nerves from their earliest condition,verifying the earlier researches of Professor A. M. Marshall,l and following the nerves to the formation of the plexuses, and their ultimate distribution in the embryonic limbs. The conclusions at which I have arrived may be summarised here; they prove conclusively the validity of the hypothesis derived from a consideration of the arrangement of the nerves in the limb plexuses of the adult. The details of my observations will be published shortly. 1. The entire length of the spinal nerves is formed by a growth of epiblastic tissue from the spinal cord. Previous ob- servers have not traced the nerves in their growth further than the junction of the motor and sensory roots. 2. At the roots of the limbs the nerves divide into dorsal and ventral trunks, which unite respectively with adjacent dorsal and ventral divisions, and can be traced to the dorsal and ventral surfaces of the limb. This part of the process has been observed in mammals as well as in the chick. 3. The formation of the nerves and the plexuses, and the passage of the nerves to the distal extremity of the limbs, occur before the differentiation of the mesoblastic tissue thereof into cartilage and muscles.

III. CONCLUSIONS. A. On the Homologies of the Spinal Nerves. The late Professor Goodsir2 states that " the nerves supplied to a limb are not the inferior or haemal divisions of the spinal nerves, but radiating or actinal branches of these divisions. The intercostal nerves are not the nerves serially homologous with the roots of the brachial plexus. The thoracic nerves, serially homologous with these roots, are the intercosto-humeral, and the succeeding middle intercosto-cutaneous." Dr Herringham3 maintains the same opinion. There is another view which I would venture to express, 1 Jour. of Anctt. and Phys., vol. xi. p. 491. 2 Edin. ANew Phil. Jour., and Anatom. Memoirs, loc. cit. 'Loc. cit. 630 DR A. M. PATERSON. namely, that the inferior division of each nerve entering a limb plexus is homologous with the whole of the intercostal or anterior lumbar nerve, and that the dorsal and ventral divisions of the nerve in the plexus correspond respectively to the lateral and anterior or inferior branches of the intercostal nerve. The evidence in favour of this view may be stated as follows:- 1. In the plexuses of mammals there are no nerves other than the ventral trunks, and the branches which they form, which can be said to represent the anterior or inferior branches of the intercostal nerves. In the brachial plexus, moreover, the first thoracic nerve always plays an important part by its axillary branch. The remaining small portion, which is intercostal, never becomes cutaneous in the ventral line of the body, but ends in the muscles of the first intercostal space. 2. In the embryo rat I have found that one of the most prominent features in the development of the spinal nerves, is the division of the entire inferior division of the nerves throughout the body into two branches, which are respectively dorsal and ventral. This division is seen in the neck, in the trunk between the limbs, and also in the regions where the limbs are being formed. The branches are in this condition of the embryo clearly homologous. The whote nerve in the regions of the limbs divides into two parts. As the complexity of parts increases, this fundamental and simple arrangement becomes masked by further changes. In snakes,' the limbs being absent, all the nerves are intercostal. They divide about the mid lateral line of the body into two equal branches for the supply respectively of the ventro- lateral and inferior aspects of the body wall. The "lateral " branch has a course downwards and backwards, and does not supply any considerable branches to the dorso-lateral surface of the trunk. A similar division of the nerves occurs in fishes,2 both in relation to the supply of the pectoral and pelvic fins, and in the trunk.

1 Swan, Comp. Anat., new syst., &c. 2 Thacker, J. K., " Median and Paired Fins: A Contribution to the History of Vertebrate Limbs," Tran2. Conreti .Acad., vol. iii., 1877; Davidoff, M. von, Beitrdge zur Vergleichenden Anatomic der hinteren Gliedmasse der FiAshe, Leipzig, 1880. THE LIMB PLEXUSES OF MAMMALS. 631 4. In Amphioxws lanceolatws,l the inferior primary division of each nerve, after sweeping round the body wall, divides, on reaching the lateral fold, into dorsal and ventral branches, which supply respectively the outer and inner surfaces of this fold. In this case, moreover, each nerve is distributed to a single myotome, and is not connected with adjacent nerves.2 As might be expected, the simplest arrangement is found in the lowest vertebrate. B. On the Origin of the Mammalian Limb. All authorities are agreed in the value of a study of neurology, in connection with the elucidation of the formation of the vertebrate limb; and of the homologies of one limb with another. This is not the place to advance or discuss any theory regarding its structure. But it is interesting to note what support the above conclusions lend to the latest theory regarding the formation of the vertebrate limb. The most recent suggestions are those of Dohrn 3 and Gegen- baur.4 While differing from one another in minor points, the main assumption of both is the same, namely, that in the most primitive condition the vertebrate limb is to be regarded as a series of separate buds, springing from the ventro-lateral aspect of the somites. These buds are supposed to merge together, and give rise to the lateral fold, as seen in Amphioxus; or the Wolffian ridge of the embryo in higher vertebrates. This lateral fold, extending along the sides of the body, disappears except at the ends, where a certain number of the fused buds remain forming the fore-limb and hind-limb. Taking these suggestions as premises, it appears that the mammalian limb is formed by the fusion of these hypothetical buds from five vertebral somites, or thereabouts (fig. 4).5 The I Rolph, W. H., "Innervation of Amphionu lanceolaw,," Morph. Jahresb., Bd. ii. hft. 1, p. 107, 1876; Untersuch. liber den Bandes Amphiox, Leipzig, 1876. 2Balfour, "Spinal Nerves in Amphioxw lanceolatus," Jotur. of Anat. and Phys., vol. X. p. 689. 3 Dolrn, Ursprtnq der Wirbelthiere, Leipzig, 1875; Studien zur Urgexh 4te des Wirbelthier KOrpers vL Mittheil. aus der Zoolog. Stat. zu Neapel, Bait Heft 1. S 4 Gegenbaur, Unters., Hft. iii.; Kopf Skcelet. der Selachier, p. 181, note, 1M7. 5 Goodsir, Opera citata. 632 DR A. M. PATERSON. primitive somites from which the buds are formed are each pro- vided with a nerve. When these have coalesced to form the lateral fold (as in Amphioxus) we find the nerves dividing in each individual myotome into dorsal and ventral branches for the supply of the dorsal and ventral surfaces of particular portions of that fold. In mammals, when the limbs are formed, as seen in the adult, there are about five nerves engaged in supplying it and forming the limb plexuses. The inference is that the mammalian limb is derived from about five vertebral somites. C. On the Formation of Limb Plexuses. From the above researches, and a comparison of the results with those of others, an adequate conception can be obtained of the mode of formation of the limb plexuses. An analysis of these plexuses in mammals shows that in the production of the nerves of " distribution," out of the nerves of " origin " two events occur. Firstly, the nerves of origin divide into dorsal and ventral trunks; secondly, these dorsal and ventral trunks subdivide and unite with the corresponding subdivisions of adjacent nerves, to form the nerves of distribution. The first step has been shown to be the result of an embryonic condition; namely, a splitting of the original nerve into trunks for the supply of the dorsal and ventral surfaces of the embryonic limb. The second step follows as a necessary result of the changes which occur in the embryonic limb, in the production of the adult condition. The bud which eventually forms the adult limb may be regarded, as stated above, as the result of the fusion of a certain number of primary buds-prolongations from the ventro-lateral aspects of certain vertebral somites. Let us suppose that five somites give such prolongations, which become amalgamated together. The primitive limb thus formed (fig. 4) presents a central core of blastema, which eventually is converted into bones and joints, surrounded above and below by layers of muscle and skin. Each myotome is supplied by a distinct and separate nerve. By the development and evolution of these primitive dorsal and ventral strata of muscles, formed of mesoblastic tissue derived from the original somites, are formed the complicated systems of muscles which exist in the adult. There is not suffi- THE LIMB PLEXUSES OF MAMMALS. 633 cient evidence as yet to enable us to trace back these muscular systems to their corresponding somatomes. Sufficient has, however, been made out by Professor Humphry' and others to show that the two muscular strata on the dorsal and ventral surfaces of the embryonic limb, become developed into certain definite systems placed superficially and deeply on the respective surfaces of the limb. These changes taking place in the myotomes, it follows that in the development of the limb similar alterations will occur in the primitive nerves which supply them. The myotomes undergo fusion, elongation, contraction, and a complex muscular system results. The nerves distributed to the original parts of this complex system must undergo similar changes; adjacent dorsal and ventral divisions becoming fused, and giving rise to a compound nerve; from which, pars pastu with the evolution of individual adult muscles, branches are given off to supply them. It follows, therefore, that as one muscle may be formed from some only of the myotomes implicated in the limb, the particular nerve for this muscle may arise only from a certain number of the nerves of origin. On the other hand, one muscle may represent the whole breadth of the surface of the primitive limb; in such a case the nerve supplying it will derive fibres from all the nerves entering the limb plexus. From these considerations, it is concluded that, as this first step in the formation of a limb plexus, the division of the nerves of origin into dorsal and ventral trunks is a primary process in the development of the limb; so the second step, the inter- connection of these adjacent dorsal and ventral divisions, is a secondary process in the same direction. The limb plexuses, that is, the formation of the nerves of distribution from certain nerves of origin by the division and union of the latter, are entirely the effect of the mode of development of the limb itself. They possess no physiological significance. There is no reason to believe that a nerve fibre divides in its course. Until the nerve breaks up into its terminal filaments there is no evidence to show that it does not exist as a simple and individual fibre from its origin in the spinal cord. In other words, it is not known that an axis 1 Observations in Myology, &c. 634 -THE LIMB PLEXUSES OF MAMMALS. cylinder divides dichotomously in the limb plexuses, so as to connect a single cell in the spinal cord with two distinct and separate muscles, or parts of the same muscle. This being the case, the existence of these plexuses is not explained by a vague reference to " the co-ordination of muscular action." The conclusion I would submit is, that they are an integral part of the process of evolution and development of the limb. They result in a convenience of nature in the adult condition. They are due to the changes which produce that condition, con- comitant with other processes, all tending to the conversion of the simple into the more complex.

EXPLANATION OF PLATE XIV. Fig. 1. Brachial, Fig. 2. Lumbar, and Fig. 3. Sacral, plexuses of the porcupine (Atherura fasiculata). Fig. 4. Diagram to illustrate the development of the limb plexuses in mammals. Joarnn. k. Voz,,Xz X/F. ofAflnaudSp1ys., Jujvl&?7 ys Vol,

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III

F. Huth Litie Bain A.M.Paterson del. T HE LI MB PLE XUSE S OF T HE PO0RC U P INE.