The M. Transversus Thoracis in Man and Monkey

Okajimas Fol. anat. jap., 48: 103-137, 1971

Jun-ichiro Satoh

Department of Anatomy, Faculty of Medicine, Nagasaki University, Nagasaki, Japan Received for publication, December 2, 1970—

Of the Mm. thoracis proprii and profundi in man, the muscles that run across the ribs in the most inner layer of the thoracic wall have been termed the Mm. subcostales by Eisler. Of these, the group of muscles located in the mid-anterior chest are the Mm. transversi thoracis while those in the posterior chest are the Mm. subcostales in the narrow sense. The purpose of the present report is to describe the M. transversus thoracis in man and monkey, especially the nerve supply to these muscles which has not yet been documented in detail. The material of study consisted of bodies of adult Japanese (24 cases), Crab-eating monkey (Macaca irus, 15 cases) and Formosan monkey (Macaca cyclopis, 15 cases). The condition on both sides of the body was examined with magnifying lenses having an illumination attachment.

Findings I. Mm. transversi thoracis in man In man, the Mm. transversi thoracis were found to be flat, narrow muscles which are multiform being quite variable in shape. In the region of the lateral edge of the sternum, especially at the xiphoid process, they were united each other so that the muscles particularly the lower ones, frequently form so-called muscular digitation, the tip being the attachment to the rib (figures 1 and 2).

1. This muscle arose by tendon from the rib in the area of the costochondral joint, that is, from the region extending from the lateral tip of the costal cartilage to the bony rib. The insertion was the lateral edge of the sternum and xiphoid process, but the condition of the muscle at the attachment was somewhat variable by site with a higher mixture of tendinous fibers for the upper muscles while the insertion was mostly muscular for the lowest muscle which attached to the xiphoid process. Thus the lowest digitation was adjacent to the M. transversus abdominis. 103 104 Juniichiro Satoh

Fig. 1. M. transversus thoracis in man (No. 524, (posterior aspect). (The muscles attached to the inner surface of the sternum)

2. In addition to the usual condition where the origin is from the area of the costochondral joint with insertion into the sternum or xiphoid process, close examination revealed instances in which the muscle arose from the intercostal space and terminated into the sternum or xiphoid process, and others in which the muscle arose from near the costochondral joint and inserted into the area adjacent to the sternocostal joint or into the rib or costal cartilage (table 1, 2, 3).

3. The Mm. transversi thoracis were located in a zone lying between the costochondral joint and the sternum. They never extended laterally (distally) beyond this area. Moreover, this is the region where the M. intercostalis internus (Eisler) ends and where the M. intercostalis intermedius (Eisler) becomes exposed as a thick muscle layer beneath the pleura together with the N. intercostalis. Thus, the position of this muscle appears to supplement the area where the M. intercostalis internus is not present. The M. Transversus Thoracis in Man and Monkey 05

Fig. 2. M. transversus thoracis in man (No. 758, (posterior aspect).

4. This muscle ran medio-downward from its costal origin to its insertion, but the angle of this muscle to the sternum was the least for the upper muscles and increasingly greater for the lower muscle.until it is almost horizontal.

5. The origin of the first muscle was sometimes as high as the first rib and the last muscle may be from the seventh rib, but such cases

Table 1. Relation between the origin and insertion in man. 106 Jun-ichiro Satoh

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were very infrequent. Usually, these muscles arose from the second to sixth ribs (table 2).

6. In addition to cases where the muscle arose from the rib as a single muscle and inserted as a single muscle, there were instances in which the insertion was separated into two parts (figure 3, A-3, D). Converse- ly, two muscles arising from the same rib may unite to insert as a single muscle (figure 3, A-2). Moreover, the insertion of the muscles into the upper part of the sternum was by a single muscle or there may be a single union of two muscles. The insertion of the lower muscles, however, particularly the muscles from the lower ribs which insert into the xiphoid process, were located more adjacent to each other with overlapping in some areas so as to occasionally present an appearance similar to a single muscle sheet. Of course, there were instances in which a space remained between adjacent muscles so that a complete muscle sheet was not formed. In contrast, at the origin these muscles were separated from each other so that they form muscular digitation which run lateralward to their site of origin from the rib. The number of muscular digitation varied, of course, depending upon the number of muscles that arose from the ribs, but there usually were five to six on each side of the body (table 3). Furthermore, the muscular digitations on each side of the body were not necessarily equal in number nor symmetrical in position.

7. The relation of this muscle to the intercostal space between its origin to insertion could be clearly determined for the muscles in the upper region but not for those in the lower region, particularly the muscle to the xiphoid process. The longer ones occupied three intercostal spaces while the shorter muscles may end in one intercostal space, but the majority occupied two intercostal spaces.

8. As mentioned above, close examination revealed that the sites of origin and insertion were variable. From the relation between the origin and insertion, it was possible to classify the muscle into four types (tables 3 and 4, figure 3). Type A: The origin is from an area extending from the lateral tip of the costal cartilage to the bony rib, in other words, the region of the costochondral joint, and the insertion is the sternum or xiphoid process. Type B: The origin is from the intercostal space with termi- nation into the sternum or xiphoid process. The M. Transversus Thoracis in Man and Monkey 113

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Type C: Origin is from the rib and insertion is into the rib or costal cartilage. Type D: The muscle arises from the rib and separates into two, parts, one inserts into the sternum or xiphoid process while the other into the rib. This can be considered to be a mixture of types A and C. Of these various types, the most common was type A. When examined by the rib of origin, types C and D tended to be comparatively more frequent among the upper muscles.

Table 4. Types of M. transversus thoracis in man observed from the sites of origin and insertion.

A muscle of type A most frequently had a single origin and a single insertion (type A-1). Other conditions seen in a few instances included the following : the muscle arose from the rib in two separate parts and united to insert as a single muscle (type A-2) the muscle was in one part at the origin but separated into two parts at the inser tion (type A-3) and the muscle was separated into two parts at both the origin and the insertion (type A-4). The last type A-4 may be considered to be the presence of two parallel type A-1 muscles from one rib (tables 3 and 4).

Nerve supply 1. The nerve supply to this muscle was by a branch from the ramus anterior of the thoracic nerve (spinal nerve), that is, the intercostal nerve. This is the same branch which innervates the Mm. intercostales intermedius and internus (Eisler). The intercostal nerves which contributed to the nerve supply ranged from the first to sixth intercostal nerve but mostly were the third to fifth intercostal nerve (table 5, figures 1 and 2). The M. TransversusThoracis in Man and Monkey 115

2. The nerve supply to this muscle was studied further, first in relation to the rib. Heretofore, the rib had been considered to be the insertion of this muscle. Nevertheless, I have attempted an evaluation based upon the rib because I believe the portion attaching to the rib should be regarded as the origin of this muscle for the reason to be described later. Moreover, at the attachment to the sternum or xiphoid process, the muscles are fused each other so that the boundary between individual muscles are indefinite which frequently made identification of the muscular unit impossible. As previously mentioned, the condition of this muscle may be classified into different types, but it was found that the relation between this muscle and the nerve supply for those in which the nerve supply could be determined may be classified and summarized as follows (tables 2 and 5, figures 1 and 2). (i) Those that arise from the rib as a single muscle (151 muscles) may be classified as follows : (a) The nerve supply is by the intercostal nerve corresponding to the rib of origin. In other words, muscle xm from the xth rib is supplied by the xth intercostal nerve (xn). This type was the most frequent one (84/151) and can be expressed as xrm.= xn. In addition to this type, there were those in which the order number of the rib of origin did not correspond with the nerve of supply. These were separated into two types. (b) More frequent was that type seen often in the muscles from the fifth and lower ribs in which the supply was by the intercostal nerve one position above. In other words, the muscle from the xth rib is supplied by the (x 1)th intercostal nerve (72/77) which can be expressed as xrm (x 1)n. (c) In the other type, which was infrequent and seen in the muscle arising from the first or second rib, the muscle from the xth rib was innervated by the intercostal nerve one position below (3/77) and can be expressed as xrm= (x + 1)n. (ii) The muscles arising from the intercostal space as a single muscle (42 muscles) were, except in only two cases, supplied by the intercostal nerve that ran in that intercostal space (40/42, xrm xn). Of the remaining cases, one was supplied by the intercostal nerve one position above (xrm (x —1)n) while the other was innervated by the intercostal nerve located one position below (xrm (x 1)n). (iii) There were muscles that arose by two heads from one rib, or muscle that at casual glance appeared to be a single muscle but actual- ly formed by the union of muscles from two sites (49). Each of these muscles could be separated into the part arising from the upper half (upper edge) of the rib and that from the lower half (lower edge) of 116 Jun-ichiro Satoh

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ゆ. 〇一 ρ 6尊 ↑ The M. TransversusThoracis in Man and Monkey 121 the rib. The nerve supply to such muscles may be classified into two types ; that in which both the portion from the upper half and the part from the lower half of the rib are innervated by the same intercostal nerve (10/49) and that type where the respective part is supplied by separate intercostal nerves (39/49). This latter pattern of nerve supply was more common. (a) In the former type, the supply was by the intercostal nerve Corresponding to the rib of origin or the intercostal nerve one position xn (x — 1)n above and may be expressed as or . In this case, — (a xn (x — 1)n horizontal line) indicates the xth rib, figures corresponding to each numerator indicate the patterns of nerve supply of the fasciculi arising from the upper half of the xth rib, and figures corresponding to each denominator show those of the fasciculi from the lower half. (b) In the latter type where the upper and lower parts arising from one rib were innervated by different intercostal nerves, the supply was by adjacent intercostal nerves with the muscle from the lower half of the rib being supplied in most instances by the intercostal nerve corresponding to the rib of origin (37/39). In other words, the muscle from the upper half of the xth rib usually received the (x —1)th intercostal nerve and the part from the lower half was supplied by the xth intercostal nerve which can be expressed as --(x—1)n . xn Among muscles of this form, there was one instance in which the part from the upper half was supplied by the intercostal nerve corre- xn sponding to the rib of origin which can be expressed as and (x 1)n another in which the muscle from the upper half of the rib was supplied by the intercostal nerve two positions above while the part from the lower half was innervated by the intercostal nerve one position above which can be expressed as (x— 2)71' . (x — 1)n

3. As mentioned, there were instances in which the nerve supply was by the intercostal nerve one position above or one position below the rib of origin. In such cases, the nerve supply to all successive muscles may be shifted one position above or below, or the nerve supply to only one muscle may be disturbed with innervation by the intercostal nerve of another segment, but in either case the intercostal nerve which belongs in that intercostal space was not absent. In other words, in the former all muscles may be regarded as having been displaced by one intercostal space. In the latter case, the 122 Jun-ichiro Satoh intercostal nerve corresponding to that rib merely sends a branch to the Mm. intercostales intermedius and internus. A branch from the intercostal nerve one position above crosses over the rib to innervate the M. transversus thoracis.

4. In the 39 cases of the type described above where the two parts of the muscle arising from one rib were supplied by separate intercostal nerves, expressed as (x 1)n (x — 2)nxnor , none showed any xn (x — 1)n (x + 1)n exchange in nerve of supply with the muscle with origin from the rib above or below (table 5). For example, examination of the nerve supply to this muscle on the left side in case J. revealed that the muscle from the upper half of the third rib was innervated by the second intercostal nerve, and the 2n —from musclethe lower half by thethird , that is, 3rrn,x1n.3 n Similarly, the nerve supply to other muscles may be expressed as : 4rm, 3n x-1 n 4n x-1 n . 4n x 5n x Thus, the portion arising from the upper half of a given rib is supplied by the same intercostal nerve as that which innervates the muscle which arises from the lower half of the next rib above. Like- wise, the muscle from the lower half of a given rib is supplied by the same intercostal nerve as the muscle arising from the upper half of the next rib below. Whenever the muscle was separated into two parts, the nerve supply of the part with origin from the upper half of the rib was the same as that of the muscle arising from the rib above even if the muscle from the rib above arose by a single head. Therefore, it is suggested that the boundary of the area supplied may reach to the muscle arising from the upper edge of the next rib below, but as a rule, it never extends beyond the midline of the rib to the lower edge of the rib or to the next intercostal space below.

5. Next, muscles which were separated into two parts at the origin from one rib (figure 3, A-2) and those which evidently were formed by the union of two muscles (figure 3, A-3, D) were each considered to be a single muscle and examined for the relationship to the nerve supply in the same way as done for those which arise from the rib as a single muscle. The results were the same as described in section 2. That is, the nerve supply to the muscle from the xth rib was pre- dominantly by-the xth intercostal nerve (167 muscles). This pattern of nerve supply is considered to be a general rule. However, the innervation by the next nerve above, that is, the (x— 1)th intercostal nerve The M. Transversus Thoracis in Man and Monkey 123 was also comparatively frequent (125 muscles). In rare cases, the supply was by the nerve two positions above, the (x —2)th intercostal nerve, or by the nerve below, the (x + 1)th intercostal nerve. Furthermore, the pattern of nerve supply appeared to be related to the position of the muscle. In other words, the fourth rib generally seemed to be a demarkation line and, as a rule, muscles from the third rib and above were supplied by the intercostal nerve corresponding to the rib of origin, the xth intercostal nerve, while the lower muscles that arise from the fifth and lower ribs were innervated by the next nerve above, the (x —1)th intercostal nerve.

6. The nerve supply to this muscle was by only the intercostal nerve corresponding to the rib of origin regardless of whether the muscle crossed over one rib to occupy two intercostal spaces or whether it crossed over two ribs and the distance from its origin to its insertion occupied three intercostal spaces as it ran obliquely downward. In other words, the muscle from the xth rib, as a rule, was innervated by only the xth intercostal nerve with no contribution by the intercostal nerve of any other segment. It is a monosegmental muscle. Moreover, the nerve supply was unrelated to the site of insertion into the sternum. Therefore, the nerve supply is determined by the rib and this is the reason for considering the attachment to the rib as the origin.

II. Mm. transversi thoracis in monkey In monkey, the Mm. transversi thoracis were extremely well developed in comparison with man. Since there was little difference in the findings for Macaca irus and Macaca cyclopis, they will be combined in the description here. Special mention will be made of only the dif- ferences (figure 4).

1. This muscle arose by tendon in most cases from both the upper and lower edges of the rib in the region extending from the costochondral joint to the costal cartilage and ran medio-downward (medio-upward in the lower muscles) to insert muscularly into the lateral edge of the sternum. Therefore, the muscle formed the muscular digitation , that is, a triangular muscular sheet with the tip being the site of origin and the base being that of insertion. During its course , it crossed over the rib but was not adhered to the rib. At the origin, there was sometimes a case in which the origin from the upper edge of the rib or that from the lower edge was lacking . The absence of the fasciculi which arose from the upper edge of the rib was frequently seen in the uppermost digitation but in the lowest digitation the absence of the fasciculi arising from the lower edge was more 124 Jun-ichiro Satoh

Fig. 4. M. transversus thoracis in monkey (Macaca cyclopis, 5-31, (posterior aspect).

common.

2. The union of adjacent muscles, particularly of the lower muscles, seen at the insertion in man was more common and more firm in monkey. Adjacent parts were united each other to form a single muscle sheet and most of them overlapped in some areas. The degree of overlapping with the next digitation was greater for the upper muscles and decreasingly in the lower part they were merely adjacent The M. TransversusThoracis in Man and Monkey 125 to each other (figure 4). As in man, there were rare cases in which a muscle arose from the intercostal space, or inserted into the costal cartilage below (3/30, figures 5-1, 2 and 4).

3. The angle formed by the course of this muscle in relation to the sternum was smaller for the muscle from the upper ribs and became increasingly greater for those from the lower ribs until it was almost horizontal.

Fig. 5. Various types of M. transversus thoracis in monkey. (posterior aspect) 126 Jun-ichiro Satoh

4. The origin of this muscle ranged from the first to seventh ribs. Most common in Macaca irus was origin from the second to sixth ribs while in Macaca cyclopis it was from the second to seventh ribs (table 6). Origin from the first rib was found in one case of Macaca irus and in two cases of Macaca cyclopis (figure 5-3), while origin from the seventh rib was absent in 17 cases of Macaca irus and in 7 cases of Macaca cyclopis.

Table 6. Range of origin of the M. transversus thoracis and its innervation in monkoy.

Note: T1... .indicates n. intercostalis I.

5. Therefore, the number of muscular apices (digitation) on each side of the body was usually five in Macaca irus and six in Macaca cyclopis Moreover, the number and location of the muscles on each side of the body were equal and symmetrical in a larger proportion of cases than in man.

6. An accurate evaluation of how many intercostal spaces are crossed by this muscle as it runs from the origin to the insertion was sometimes difficult because, as mentioned above, the muscles unite to form a muscular sheet in monkey. However, inspection of the course of the muscle bundles revealed that the muscle generally crosses over one rib The M. Transversus Thoracis in Man and Monkey 127 to occupy two intercostal spaces except in rare cases in which it occu- pies three intercostal spaces (figure 5-4).

7. The muscles (muscular digitation) are adjacent to each other at the insertion to form a muscular sheet. There usually was no area of interruption between these muscles except in rare cases (1 case each in Macaca irus and Macaca cyclopis, figure 5-3). Furthermore, the inspection in the lowest digitation revealed that in some cases the muscular sheet was separated into the superficial layer and the deep layer near the site of insertion. The group of muscle bundles (sheets) of superficial one inserted into the sternum and the muscle bundles (sheets) of deep one terminated into the costal cartilage one position below (Macaca cyclopis, No. 23, figure 5-4). At the insertion, the muscle usually converged on an area on the sternum corresponding to the width of a intercostal space. Therefore, fasciculi arising from one rib inserted into the sternum or extended a little further downward. In the lower region, the muscle bundles arising from two adjacent ribs sometimes inserted into the area of one sternum. Although this muscle usually never extended to the first rib, the uppermost muscular apex extended as far as the first intercostal space and attached to the manubrium in 4 cases of Macaca irus (G, H, figure 5-2).

8. The location of the M. transversus thoracis was the region where the M. intercostalis internus (Eisler) is absent. In other words, the site of origin of this muscle never extended laterally beyond the costochondral joint. Thus, the location of this muscle appears to supplement the area where the M. intercostalis internus is absent.

• Nerve supply

1. The nerve supply to this muscle was similar to that in man. It was by a branch from the ramus anterior of the spinal nerve, in other words, the intercostal nerve which entered the muscle from its lower surface. The nerve supply suggests that this muscle belongs to the same system as the Mm. intercostales intermedius and internus (Eisler) . Furthermore, the nerves that contributed to the nerve supply were the first to sixth thoracic nerve in both Macaca irus and Macaca cyclopis . In particular, contribution by the second to fifth thoracic nerve was the most frequent (table 6, figures 4, 6 and 7).

2. As mentioned earlier in the section on the results , this muscle formed a triangular muscular sheet (digitation) with the tip being the 128 Jun-ichiro Satoh site of origin at the rib and the base being that of insertion into the sternum. Generally, part of the muscular sheet in the upper region was overlapped with the next muscular sheet below. Close examination of the area supplied by a branch of any one intercostal nerve revealed that the area supplied was not only the muscular digitation arising from the rib to which the intercostal nerve corresponded but it extended to almost as far as the upper half of the next muscular digitation below. For example, in the figure 6 the first muscular digitation, the tip of which is the second rib, was distributed and innervated by a branch of the second intercostal nerve, but the branch extended not only to this first muscular digitation. but also farther to the upper half of the second muscular digitation, that is, the fasciculi which arose from the upper edge of the third rib. Similarly,

Fig. 6. Innervation of the M. transversus thoracis in monkey (Macaca cyclopis). (posterior aspect) The M. Transversus Thoracis in Man and Monkey 129 the area innervated by the branch of the fourth intercostal nerve was the lower half of the third muscular digitation which mainly arose from the lower edge of the fourth rib and the upper half of the fourth muscular digitation arising generally from the upper edge of the fifth rib (figure 6). Thus, from a morphological point of view, each muscular digitation seen as a unit receives branches from two separate intercostal nerves so as to present a complicated picture of double innervation. Close inspection, however, revealed that the area supplied by any one intercostal nerve seems to be mostly limited to the area within one intercostal space. In other words, the area supplied by the xth intercostal nerve was the muscle group arising from the lower edge of the xth rib and the upper edge of the (x + 1)th rib . Thus, the muscular unit from a view- point of nerve supply is roughly the area between adjacent ribs or the intercostal space. From such a standpoint, the pattern of nerve supply to this muscle may be said to be a regular, segmental distribution similar to that for the M. intercostalis (table 6). The finding that fasciculi arising from the area of one intercostal space in monkey generally inserted into only an area of the width of one sternum provided further proof in support of my previously mentioned assumption.

Fig. 7. Abnormal type of innervation (Macaca cyclopis , No. 23, ), (posterior aspect). 130 Jun-ichiro Satoh

3. There were, however, rare cases among both Macaca irus and Macaca cyclopis in which the fasciculi were supplied by the intercostal nerve in the next intercostal space above (figure 7). This condition was not in agreement with the above mentioned rule that the muscle group arising from the area of one intercostal space is innervated by the intercostal nerve that runs in that intercostal space, but such cases should be considered to be a variation in which part of the fasciculi from the next rib above separated and was displaced downward to attach to the rib below.

Discussion 1. The M. transversus thoracis of mammals is reported to be better developed than that in man (Le Double). Its condition in primates is said to differ little from that in man (Kohlbrugge), but my finding revealed that it is much better developed than in man. Moreover, in monkey, fasciculi are adjacent to each other to form a muscular sheet, and further, concerning these muscular sheets which form each muscular digitation after arising from each rib, adjacent muscles at the insertion into the sternum are overlapped with each other or formed a successive muscular sheet. In man, a muscular sheet may be formed, but its extent is limited and the muscular sheet formed is not necessarily completely continuous and component muscles sometimes may have a space between them or they may even be con- siderably widely separated. Furthermore, in most cases the portion. near the sternum in man is mixed with tendinous fibers or is aponeuro- tic. The increase of tendinous elements in the M. transversus thoracis of man has also been mentioned in the study of Erythrocebus by Virchow who considered this to be a retrogressive phenomenon. Thus, my findings for monkey and man clearly indicate a retrogressive phenomenon.

2. Moreover, in addition to the usual condition where the origin is a rib or a costal cartilage, close examination revealed some instances in which the muscle must be considered to arise from the intercostal space. However, since the supply to these muscles is, as a general rule, by the same nerve as that to the muscle arising from the rib, the site of origin from the rib in a case of the digitation which arose from the intercostal space is, owing to degeneration or fibrosis, hardly recogni- zable macroscopically so that it seems at casual glance to have regressed until it disappeared.

3. The location of the first muscular digitation in relation to the rib The M. Transversus Thoracis in Man and Monkey 131 was in most cases the second rib in both man and monkey in my inves- tigation. According to the statistics of Mori on Japanese, it most frequently was the third rib while in the study on Poles by Loth, it was the second and third rib at about equal frequency (table 7). Therefore, in comparison with the statistics of Mori, a slightly higher frequency of the primitive type of Loth was noted in my findings, but this difference presumably is due to the small number of cases in my series. In primates, the site of attachment of the first muscular digitation is considered to be the second rib in prosimian and platyrrhine monkey, the second or third rib in catarrhine monkey and gibbon, and the third rib in gorilla, orangutan and chimpanzee. The findings in my cases of catarrhine monkey correspond to those seen in the former literature.

Table 7. The rib of the origin of the uppermost and the lowest digitation of the M. transversus thoracis.

The rib from which the last muscular digitation arose in man was usually the sixth rib in my study which is in agreement with those of Mori and Loth. In monkey, it was frequently one rib lower and particularly in Macaca cyclopis, it was as a rule the seventh rib. Therefore, the location of these muscles in man is more limited with confinement from both above and below as compared with monkey so that it is localized at a more restricted area. 132 Jun-ichiro Satoh

Loth has classified the first muscular digitation into five types according to the level of the rib of attachment and the last muscular digitation into three types. Among the former, he considered origin from a high rib, and, among the latter, origin from a low rib to be the primitive type. Even in my cases, this exact relation was demon- strated. The presence of this muscle in monkey over a wider area and the confinement of this muscle in man from both above and below perhaps may be a retrogressive phenomenon which is related with the finding that the Mm. intercostales interni in man are more restricted in their location than in monkey with a gradual disappearance and absence of these muscles occurring from both the upper and lower , intercostal spaces. Moreover, the upper muscular digitations are smaller which may be evidence that this retrogressive phenomenon begins from above and below. Further, the number of muscular digitation formed by this muscle was five in man and six in monkey which agrees with the findings of Loth and others.

4. (i) The nerve supply to this muscle was by the intercostal nerve. The range of spinal nerves which contribute to the supply in man was the first to seventh thoracic nerve, most frequently by the second to fifth thoracic nerve, which agrees with the description by Eisler. The condition in monkey was similar to that in man except that the range is more extensive than in man. Contribution by a more extensive range of nerves to this muscle in monkey suggests that it has origin from a wider range of ribs and that it is formed by broader muscular material. (ii) The muscle from the xth rib and that from the xth intercostal space in man, as a rule, was supplied by the xth intercostal nerve. As mentioned in man, however, there were many instances where the first muscle arising from the first or second rib is supplied by the next nerve below, that is by the (x + 1)th intercostal nerve, while those with origin from the fifth rib and below are innervated by the next nerve above, that is the (x-1)th intercostal nerve. In monkey, the xth intercostal nerve innervates the muscle bundles arising from the lower edge of the xth rib and the upper edge of the (x+ 1)th rib, in other words, it supplied the fasciculi which arise from the region of the xth intercostal space. Therefore, it is found clearly in monkey and in a large majority cases in man that the relation between the nerve and the associated muscle maintains the primitive type of segmental structure and arrangement. In addition to this, in many cases in man, however, in the upper part the muscular material seems to be displaced one inter- The M. Transversus Thoracis in Man and Monkey 133 costal space upward to attach to the rib above, while in the lower region the muscular material is displaced downward by one intercostal space to seek their attachment into a lower rib. (iii) In man, in addition to cases with a single muscle arising from one rib, there were instances in which at casual glance the muscle appears to be a single muscle, but careful inspection shows it to be formed by the union of two muscles, one from the upper half of the rib and the other from the lower half of the rib. Each of these had their own intrinsic nerve supply and although both may sometimes be innervated by the branch from the same intercostal nerve they usually were supplied by separate intercostal nerves. Moreover, the supply to the muscle from the lower half was by the same nerve as that to the muscle arising from the upper half of the next rib. Therefore, this indicates that all the muscles arising from the lower half of a rib and the muscle from the upper half of the next rib are supplied by the intercostal nerve which runs in the intercostal space. In other words, a line almost along the middle of the rib is the boundary in which the region bounded by the midlines through two adjacent ribs is the area supplied by one intercostal nerve and this condition is the same as that for the M. intercostalis which is supplied by the intercostal nerve run- ning in that intercostal space. (iv) On the other hand, in monkey the muscle is well developed and forms a triangular muscular sheet, with the tip being the site of origin and the base being that of insertion into the sternum, which should be considered to be the muscular unit. However, the fasciculi arising from the lower edge of the rib and from the upper edge of the next rib below, in other words, from the area of one intercostal space, should be regarded to be the unit of innervation since, as mentioned, all the fasciculi arising from the area within this intercostal space are supplied by the same intercostal nerve and, further, that the site of the rib where the fasciculi arise is the upper or lower edge when a line along the middle of the rib is the boundary determines the nerve which supplies. Therefore, the condition (pattern) of nerve supply in both man and monkey is the same. (v) In man, for muscles that arise from one rib by two heads or which are definitely formed by the union of two muscles, it was mentioned that they may be supplied by the same nerve in some instances and by separate intercostal nerves in others. The former condition is considered to be separation into two parts of what originally had been one muscle derived from one muscular material. In the latter case, the upper muscle is derived from material of the muscle associated with the intercostal nerve one level above the lower muscle, in other words 134 Jun-ichiro Satoh

theupper and lower muscles were originally derived from separate adjacent material, and this indicates the displace of position of the muscular material. (vi) In man this muscle was supplied by only the intercostal nerve from one segment regardless of the length of the muscle whether the distance from the origin to insertion extended over only one rib or two ribs so that it is a monosegmental muscle. Moreover, the nerve supply to the muscle is determined by the rib from which it has arisen and not by the site of insertion into the sternum or xiphoid process. In other words, the nerve supply to the muscle is determined by the rib to which the muscle is attached an unrelated to the site of insertion by the other end into the sternum or xiphoid process. It is entirely the same in monkey that the nerve supply to this 'muscle is determined by the rib from which the muscle arises. This is the reason that I considered the attachment to the rib to be the origin of this muscle which is in contrast to the past method of considering the origin and insertion of this muscle. (vii) The nerve supplying this muscle is the same branch that innervates the Mm. intercostales internus and intermedius. Moreover, its location appears to compensate the area in which the M. intercostalis internus is absent and is never adhered with the M. intercostalis intermedius so that it seems that this muscle belongs to the same system as the M. intercostalis internus. However, there were some cases (No. 37 and others) in which this muscle was given off not only from the intercostal space where the M. intercostalis internus was absent but from the M. intercostalis intermedius located there and moreover their nerve supply was the same. This seems to indicate that in addition to cases where this muscle is derived from the M. intercostalis internus there are rare instances in which it is derived from the M. intercostalis intermedius. (viii) Presumably, this muscle belongs to the same system as the M. intercostalis internus and in rare instances belongs with the M. intercostalis intermedius or it may even be part of the M. intercostalis internus in a broad sense. That is, one part of the M. intercostalis after arising from the rib is elongated and, instead of inserting into the next rib, extends beyond it to insert into a rib farther down (which seems to be the condition in case number man-T, right side) after which it then changes its course to attach to the lateral edge of the sternum or xiphoid process. Furthermore, since this muscle is well developed in monkey, adjacent muscles unite each other to form a muscular sheet. In man, however, the difference from monkey is only that a muscular sheet is not formed, or the formation of the muscular sheet, which is The M. Transversus Thoracis in Man and Monkey 135 the primitive type, may have retrogressed and disappeared in some areas with evolution and the formation of a space, separation, etc. noted occasionally in man, particularly in the lower half presumably indicates part of this retrogressive process.

Summary

1. The M. transversus thoracis in both man and monkey arises from the rib in the area extending from the lateral tip of the costal cartilage to the bony rib, that is, from the rib or costal cartilage near costochondral joint, and inserts into the lateral edge of the sternum or xiphoid process. In man, however, there were instances in which this muscle arose from the intercostal space, that in which it inserted into the sternocostal joint, etc., so that depending upon the relation between the origin and insertion a classification into four types was possible. A further subdivision was possible according to whether the origin or insertion is by a single muscle or separated into two parts.

2. The rib of origin in monkey ranges from the first to seventh rib, but the origin from the first rib was frequently absent. In man it was more limited both above and below, usually being from the second to sixth rib.

3. This muscle in monkey was well developed so that at the insertion the muscular digitation united each other to form a triangular muscular sheet with the tip being the site of origin at the rib and the base being that of insertion into the sternum. In man, however, fusion at the insertion and thus the formation of a muscular sheet was found only for those muscles arising from the lower ribs.

4. This muscle in man and monkey mostly crossed over one rib to occupy two intercostal spaces.

.5. The location of this muscle corresponds to the area in which the M. intercostalis internus is absent so that it appears to compensate the region where the M. intercostalis internus is absent.

6. The nerve supply to this muscle is by a branch of the intercostal nerve which is the same branch that supplies the Mm. intercostales internus and intermedius (Eisler). Furthermore, the thoracic nerves which contribute to the nerve 136 Jun-ichiro Satoh supply in man are usually the third to fifth intercostal nerve while in monkey it is the second to sixth intercostal nerve so that the contri- buting nerves in man are more restricted from above and below.

7. The nerve supply to this muscle in man is usually by only one intercostal nerve which corresponds to the rib from which this muscle arises. The intercostal nerve of another segment never contributed. regardless of the length or site of insertion of this muscle. Conse- quently, since this is a monosegmental muscle and the nerve supply to this muscle is determined by the rib to which the muscle is attached, it is felt that the attachment to the rib should be regarded as the origin.

8. The pattern of nerve supply in man and monkey was by the intercostal nerve corresponding to the rib of origin of this muscle, in other words, the muscle from the xth rib was supplied by the xth intercostal nerve, but in man the muscles located in the upper anterior chest were supplied by the intercostal nerve one position lower while the muscles in the lower were supplied by the intercostal nerve one level above. Thus, in man the original segmental arrangement of the muscle and nerve is maintained, but those in the upper and lower parts should be regarded as having been displaced by one intercostal space upward or downward. For the two muscles which arise from one rib (the xth rib), the muscle arising from the upper edge is supplied by the next intercostal nerve above (the (x— 1)th intercostal nerve), and the other muscle from the lower edge by the nerve corresponding to the rib (the xth intercostal nerve). Similarly, the muscle arising from the upper edge of the next rib below (the (x+ 1)th rib) is usually innervated by the xth intercostal nerve. Therefore, the area supplied by one intercostal nerve (the xth intercostal nerve) is the region bounded by the midline through two adjacent ribs, in other words, roughly speaking, the muscle bundles arising from the area of one intercostal space.

9. In monkey, one muscular digitation, when the digitation arising from one rib was considered to be a unit, was supplied by two different intercostal nerves. The area supplied by one intercostal nerve, that is, the area innervated by the xth intercostal nerve was the muscle bundle arising from the lower edge of the xth rib and that arising from the upper edge of the (x+ 1)th rib so that the area almost corresponded to the muscle bundles arising from the area of one intercostal space. Therefore, the unit of nerve supply is the muscular bundles arising from one intercostal space. The M. Transversus Thoracis in Man and Monkey 137

Therefore, the patterns of nerve supply in both man and monkey are entirely the same.

10. In view of the relation between the location of this muscle and the M. intercostalis internus, and the condition of nerve supply, it must be said that this muscle belongs to the same system as the M. intercostalis internus in the general sense and although it usually is derived from the M. intercostalis internus (Eisler), it may rarely be derived from the M. intercostalis intermedius.

11. In considering the course of formation of this muscle, it seems that the muscle bundle after separating and differenciating from the same material as the M. intercostalis internus extended downward instead of inserting into the next rib below like the M. intercostalis internus, and crossed over one or two ribs to insert into a rib farther down, followed then by a change in course to insert into the sternum or xiphoid process.

References Le Double 1897. Traite des variations du systeme musculaire de l'Homme. Paris. Eisler, P. 1912. Die Muskeln des Stammes. Jena. Foster, A. 1917. Zur Morphologie der Muskeln des Thorax. Morph. Anthrop., 20. Hartman and Straus 1961. The Anatomy of the Rhesus Monkey. New York. Kohlbriigge. I.H.F. 1897. Muskeln und Periphere Nerven der Primaten, mit besonderer Beriicksichtigung ihrer Anomalien. Amsterdam. Kolesnikow, W.W. 1930. Ober die Mm. subcostales der Sangetiere. Zeit. Ges. Anat. I. Abt., 92. Loth, E. 1931. Anthropologie des Parties Molles. Paris. Mori, M. 1964. Statistics on the Musculature of the Japanese. Okajimas Fol. anat. jap., 40. Stewart, T.D. 1936. The musculature of the anthropoids. I. Neck and trank. Amer. J. Phys. Anthrop., 21. Virchow, H. 1915. Intercostalis externi und transversus thoraco-abdominalis des Ery- throcebus patas. Archiv. f. Anat. u. Phys. Anat. Abt. Suppl.