The Rotatory Mobility of the Fibula in Eutherian Mammals by C

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THE ROTATORY MOBILITY OF THE FIBULA IN EUTHERIAN MAMMALS BY C. H. BARNETT AND J. R. NAPIER Department of Anatomy, St Thomas's Hospital Medical School It is generally recognized that the rotatory mobility of the fibula varies considerably throughout the eutherian mammals. Little functional significance has been assigned to this variation, however, apart from the observation of Carleton (1941) that immobility of the fibula is characteristic of jumping, burrowing and swimming mammals, while mobility is confined to certain members of the Carnivora and the Primates. Walmsley (1918) came to the conclusion that the proximal end of the fibula existed mainly as a muscular process, while the distal end was the significant portion which owed its existence in a mobile form to the movements of the foot. Owen's view (1866) was similar, though more specific, when he stated 'it is to this prehensile power [of the foot] that the modifications of the fibula chiefly relate'. The authors (1952) have shown that, in man, the mobility of the fibula, as judged by the criteria of the shape, inclination and size of the superior tibio-fibular joint, is related to movement at the ankle and hence to the form of the talus. The fibula is mobile in those individuals in whom the axis of rotation at the ankle joint is steeply inclined, but is relatively immobile if this axis is horizontal. This finding in man suggests a method of approach to the wider problem of the functional significance of fibular mobility in other mammals. In the present investigation an attempt has been made to link mobility of the mammalian fibula with movements at the ankle during locomotion.

MATERIALS AND METHODS Ninety-four species, comprising representatives from all the known orders of eutherian mammals with the exception of the Cetacea, Sirenia and Dermoptera, and including most of the important families, were examined. The specimens were, for the most part, in the form of disarticulated limb bones, but at least one wet specimen from each of the more common orders and suborders was dissected; these amounted to twenty-four in all and are indicated in Tables 1-3. Where rotatory mobility was anticipated the bones were stripped of all their muscles, leaving a ligamentous preparation of the two limb bones with the attached foot. Metal indicator pins were driven into each long bone and the relative divergence of these two pointers during passive movement, indicating rotation of the fibula relative to the tibia, was photo- graphed and measured. This rotation is usually extremely small, and it may easily be missed unless such a procedure is adopted. Measurements of the tali were made in accordance with the method employed for the human talus. The axis of the ankle joint was ascertained by finding central points of the circles of which the curvatures of the medial and lateral profiles of the talus form a part. 12 C. H. Barnett and J. R. Napier

OBSERVATIONS The majority of fibulae could be classified according to the characteristics of the superior and inferior tibio-fibular joints into three distinct groups: the immobile, the mobile and the intermediate. (1) The immobile fibula, partly or wholly fused to the tibia, is widely represented among the mammals, and examples were found in all the orders examined, although the tarsier, Tarsius spectrum, is unique among the Primates (Table 1). There are Table 1. The immobile fibula (Where wet specimens have been examined they are indicated by an asterisk.) Superior Inferior tibio-fibular tibio-fibular Animal Order joint joint *Erinaceus europaeus (hedgehog) Insectivora Fused Fused Gymnura gymnura (gymnura) Insectivora Fused Fused *Sorex araneus (common shrew) Insectivora Synovial Fused Desmana moschata (Russian desman) Insectivora Fused Fused Potamogale velox (otter shrew) Insectivora Synovial Fused Macroscelides probosctdeus (common Insectivora Synovial Fused elephant shrew) Petrodromus tetradactylus (four- Insectivora Fused Fused toed elephant shrew) Rhynchocyon cirnei (long-nosed Insectivora Fused Fused jumping shrew) Centetes ecaudatus (tenrec) Insectivora Synovial Fused *Chrysochloris asiatica (cape golden Insectivora Fused Fused mole) Halichoerus grypus (grey seal) Carnivora Fused Syndesmosis Otaria californiana (sea-lion) Carnivora Fused Syndesmosis Procavia capensis (Cape coney) Hyracoidea Fused Fused P. syriaca (Syrian coney) Hyracoidea Fused Fused P. abyssinica (Abyssinian coney) Hyracoidea Fused Fused Dendrohyax dorsalis (Gold Coast Hyracoidea Fused Syndesmosis tree-coney) *Rattus norvegicus (bwown rat) Rodentia Synovial Fused *Mus musculus (house mouse) Rodentia Synovial Fused Bathyergus maritimus (African Rodentia Fused Fused sand-mole) *Cavia cobaya (guinea-pig) Rodentia Synovial Fused Jaculus jaculus (African jerboa) Rodentia Synovial Fused *Oryitolagus cuniculus (rabbit) Lagomorpha Synovial Fused Lepus europaeus (hare) Lagomorpha Synovial Fused Priodontes gigas (giant armadillo) Edentata Fused Fused Orycteropus afer (aard-vark) Tubulidentata Fused Syndesmosis *Tarsius spectrum (tarsier) Primates Synovial Fused

two main varieties within this group: first, those showing bony fusion of the tibia and fibula at the upper end associated with fusion or an extensive fibrous tissue union at the lower (Figs. 1 and 2); and, secondly, those showing fusion at the lower end with a synovial joint at the upper (Fig. 3). Both types are associated with a talus the appearance of which is generally rather square, there being no splaying of the medial and lateral sides (Fig. 4A). The two profiles of the trochlear surface of the bone often show symmetrical and equal curvatures, so that the axis of movement at the ankle tends to be parallel to the plane of the trochlear surface and a true ginglymoid movement results. It may be seen from Table 1 that within the same zoological order the tibio-fibular articulations may show quite different characteristics. For example, the fused upper The rotatory mobility of the fibula in eutherian mammals 13 and lower tibio-fibular joints in the African sand-mole, Bathyergus maritimus, appear to make this species unique in this respect in its own particular order. Among the Hyracoidea, a synostosis at the upper end and a syndesmosis at the lower are typical of tree-livingvarieties, while asynostosis at bothends isfoundamongthefossorial coneys. On the other hand, similar tibio-fibular articulations are found in members of quite different orders; for example, a synovial joint at the upper end of the fibula with a

Fig.*l Fig. 2 Fig. 3

Fig. 1. Cape golden mole (Chrysochloris asiatica). x 5. Fig. 2. Grey seal (Halichoerus grypus). x i. Fig. 3. African jerboa (Jaculus jaculus). x Ii.

A B Fig. 4. Diagrams illustrating the relation of the axis at the ankle joint to the lateral articular surface of the talus. A, tali found in association with the immobile fibula-the axis may or may not be parallel to the upper surface; B, talus usually found in association with the mobile fibula. synostosis at the lower is found among many Insectivora (e.g. the European mole, Talpa europaea), in most members of the Rodentia and Lagomorpha (e.g. the jerboa, Jaculus jaculus, and the common rabbit, Oryctolagus cuniculus), and within the Primates (the tarsier, Tarsius spectrum). These observations suggest that identical forms of tibio-fibular articulation in diverse mammalian types are the results of functional adaptations and are independent of taxonomic considerations (Fig. 11). (2) The mobile fibula, where both upper and lower tibio-fibular joints are synovial, is found in only two orders, the Carnivora and the Primates (Table 2). 14 C. H. Barnett and J. R. Napier

Table 2. The mobile fibula (Both the superior and the inferior tibio-fibular joints are synovial in all the species listed in the table. Where wet specimens have been examined they are indicated by an asterisk.) Order: Primates Order: Carnivora *Lemur catta (ring-tailed lemur) *Felis domesticus (domestic cat) L. fulvus (brown lemur) Panthera leo (lion) L. variegatus (ruffed lemur) *P. tigris (tiger) *Perodicticus potto (common potto) P. pardus (leopard) Cebus capucinus (capuchin) Lynx lynx (lynx) Ateles frontatus (spider monkey) *Lutra lutra (otter) *Macaca mulatta (Rhesus monkey) *Ailuropoda melanoleuca (giant panda) M. speciosa (stump-tailed macaque) *Selenarctos tibetanus (Himalayan M. nemestrina (pig-tailed macaque) black bear) Papio hamadryas (sacred baboon) Ursus arctos (brown bear) *P. cynocephalus (yellow baboon) Thalarctos maritimus (polar bear) P. anubis (anubis baboon) P. porcarius (chacma baboon) Mandrillus sphinx (mandrill) Hylobates hoolock (gibbon, hoolock) H. lar (gibbon, lar) Simia satyrus (orang-utan) *Pan troglodytes (chimpanzee) Gorilla gorilla (gorilla)

In the twenty-nine species examined the upper articulation consisted of a large synovial joint, the lower of a short syndesmosis and, in addition, a large semilunar facet on the tibia covered by articular cartilage and in continuity with the ankle joint (Figs. 5 and 6). Rotatory mobility of this type of fibula was demonstrated by means of metal indicator pins driven into the shafts of the leg bones 1,, / in six wet specimens (chimpanzee, baboon, lemur, bear, tiger and cat) stripped of their muscles with their ligaments left intact. In chimpanzee, lemur and bear, the mobile phase of fibular movement occurs as the foot is passively dorsi-flexed from the neutral position. Complete separation of the bones is prevented by the tension in the posterior talo-fibular and transverse tibio- fibular ligaments, while the anterior talo-fibular and tibio-fibular ligaments are relaxed, allowing the fibula to hinge posteriorly. The movement of lateral / rotation can be demonstrated after all the ligaments have been divided provided the fibula is held in Fig. 5 Fig. 6 contact with the side of the talus-thus indicating Fig. 5. Orang-utan, Simia satyrus. that the main factor in producing the movements is x 1. the conformation of the lateral side of the talus. Fig. 6. Himalayan bear, Selen- aretos tibetanus. x -. In the cats and the baboon, on the other hand, fibular rotation is limited to the plantar-flexion phase, the bone undergoing a medial rotation as the foot is plantar-flexed from the neutral position. In the cat, the anterior ligaments remain taut and the hinge around which the fibula rotates is therefore placed anteriorly. The rotatory mobility of the fibula in eutherian mammals 15 The range of movement in all species is small and does not exceed 7 degrees, the greatest mobility occurring in the cat family. Other movements of the fibula have been observed but they appear to be of secondary importance to rotation. In this group, the sides of the talus are often divergent; this is especially marked in the larger Primates and the Ursidae (Fig. 4B). (3) The intermediate fibula. All the species so far mentioned have either an im- movable joint at one or other end of the fibula, thus eliminating all rotatory movement, or mobile joints at both ends in which case rotation is clearly possible. There is, however, a further group in which rotational movements can theoretically occur. A synovial tibio-fibular joint is found superiorly, and a syndesmosis, commonly associated with a small synovial cavity, inferiorly. This group consists of a few members of several orders (Table 3), including a single representative ofthe Primates: the tree shrew, Tupaia belangeri (Fig. 7). In the Canidae, the syndesmosis is large,

Table 3. The intermediate fibula (All the species listed in the table have a synovial joint at the upper end and a syndesmosis at the lower end, often associated with a small synovial recess. Where wet specimens have been examined they are indicated by an asterisk.) Species Order Crocuta crocuta (spotted hyaena) Carnivora *Canis familiaris (domestic dog) Carnivora *Vulpes vulpes (common fox) Carnivora Meles metes (badger) Carnivora Hylochoerus meinertzhageni (African forest hog) Artiodactyla Peccari tajacu (collared peccary) Artiodactyla *Sus scrofa (domestic pig) Artiodactyla Hippopotamus amphibius (hippopotamus) Artiodactyla Rhinoceros unicornis (Indian rhinoceros) Perissodactyla Diceros simus (white rhinoceros) Perissodactyla Tapirus indicus (Malayan tapir) Perissodactyla T. bairdii (Baird's tapir) Perissodactyla Elephas maximus (Indian elephant) Proboscidae Sciurus vulgaris (red squirrel) Rodentia Castor fiber (beaver) Rodentia Myrmecophaga tridactyla (great anteater) Edentata Tupaia belangeri (tree-shrew) Primates

extending for at least one-third of the length of the shaft. Since by the study of the dry bones the possibility of rotatory mobility could not be excluded, metal indicator pins were driven into the leg bones of ligamentous preparations in two species, the domestic pig and the dog (three specimens). In neither species could any rotation be demonstrated on passive movements of the foot. It has been noted in some species with this type of tibio-fibular articulation that the joints tend to ossify in old age, e.g. hippopotamus and tapir. Highly specialized types. The functional requirements of certain species result in a highly specialized hind-limb leading to radical changes in the relationship of the fibula to the tibia and tarsus, and in the form of the talus. Among the Chiroptera, the fruit bat, Pteropus aegypticus, shows an ankle joint with a ball-and-socket mechanism to permit adduction and abduction of the foot, and a greatly reduced fibula. Among the Edentata, the collared sloth, Bradypus torquatus, possesses a ball-and-socket articulation between the lower end of the fibula and the talus; 16 C. H. Barnett and J. R. Napier this specialization is presumably related to the unique mode of progression of this animal, the limbs being used to support the weight of the dependent trunk. The Artiodactyla, with the exception of the few species referred to above, have lost the upper part of the fibula but have retained the lower end in the form of an os malleolare. These modifications may be correlated respectively with the absence of inversion and eversion of the foot rendering a large bony origin for the peronei redundant, and with the necessity for maintaining a stable hock joint. The talus

Fig. 7 Fig. 8 Fig. 9 Fig. 7. Tree shrew, Tupaia belangeri. x 1i. Fig. 8. Malayan tapir, Tapirus indicus. x Fig. 9. Sheep, Otis aries. xi. is markedly trochleariform and the lateral side consists of a deeply concave articular facet for reception of the os malleolare. In two of the seventeen specimens examined, the cow, Bos taurus, and sheep, Ovis aries (Fig. 9), experiments were carried out to determine mobility of the os malleolare in wet specimens. The only movement that could be detected was a very small amount of splaying of the bone around an approximately horizontal hinge in full plantar-flexion. No axial rotation was demonstrated. In Equidae the lower end of the fibula is completely incorporated into the tibia and there is no true lateral malleolus. In these animals stability of the ankle joint is assured however by means of the closely interlocking nature of the bones, rendering the presence of a lateral buttress inessential.

DISCUSSION Two types of tibio-fibular articulation have been found associated with the immobile type of fibula: (a) A slender, flexible fibula with an upper synovial joint and a synostosis at the lower end, which in many species may be actually incorporated with the tibia. The functional significance of this type of fibula is obscure. Its extreme flexibility, however, suggests that distortion of the bone may occur in the initial phase of con- The rotatory mobility of the fibula in eutherian mammals 17 traction of the attached muscles during the period when the inertia of the ankle joint and the tone of the antagonist muscles is still being overcome. Greater initial velocity results when joint movement is finally brought about by the fully contracted muscle. The action of the fore-finger and thumb in flicking a pellet is a familiar example of a similar mechanism, joint movement being held back until the agonists are fully contracted and the antagonists fully relaxed. This hypothesis receives support from the observation that this type of fibula is present in those creatures which are capable of sudden, rapid movements of jumping and bounding such as the hare, the jumping shrew, the jerboa and the tarsier. It is of interest that a similar tibio-fibular articulation is found in the saltatorial marsupial, the red kangaroo. (b) A stout, inflexible fibula fused to the tibia at the upper end and united at the lower end by bone or by fibrous tissue is characteristic of the swimming or burrowing mammals, for example, the seal, the cape golden mole, the desman and the armadillo. This arrangement, which provides a rigid origin for muscles acting on a free foot, is requisite for swimming and burrowing activities where there is no distal fixed point of leverage such as is provided by the ground during terrestrial progression. The mobilefibula is the less common type, and is virtually limited to cats and bears among the Carnivora and to the majority of Primates. The fibula articulates with the tibia superiorly by a large horizontally directed synovial joint and inferiorly by a synovial joint which is typically semilunar in shape; thus fibular rotation is theoretically possible throughout the range of ankle joint movement, although in practice it is usually confined to a certain phase. All these animals are either remarkably well adapted to progression, often at high speeds, over rocky and uneven surfaces (e.g. cats, bears, gorillas and baboons) or are wholly or partly arboreal (e.g. orang and chimpanzee). Both these modes of life are similar in that they not only demand a wide range of flexion and extension at the ankle but also tend to promote inversion and eversion strains in the same joint. MacConaill (personal communication) has shown that a certain amount of axial (conjunct) rotation must occur at all hinge-joints in order to provide efficient lubrication. This effect is dependent upon the existence of an obliquely-placed axis of rotation. The evidence for this oblique axis at the ankle joint in this group of Primates and Carnivora is provided by the unequal curvatures of the medial and lateral sides of the talus, as has already been shown in man (Barnett & Napier, 1952). In these animals the lateral side of the talus is thus not perpendicular to the axis of rotation (Fig. 4). It necessarily follows that, as the talus is dorsi-flexed, the plane of the lateral side changes, in accordance with the principle that unless a rotating surface is perpendicular to the axis about which it is moving its plane cannot remain constant. The fibula must undergo a corresponding lateral rotation if the adjacent surfaces are to remain in contact. Since the fibula is thick and inflexible such rotation is possible only in the presence of movable joints at both upper and lower ends. In general, the degree of conjunct rotation in a hinge-joint is proportional to the functional range of movement. In this group of mammals the range of ankle movement is great (Young, 1950), and thus if contact between the talus and fibula is to be maintained-and this is essential in animals which are subject to constant inversion and eversion strains-the fibula must be adjustable. Fibular mobility is Anatomy 87 2 18 C. H. Barnett and J. R. Napier therefore essentially a mechanism to stabilize the ankle joint in animals whose way of life depends on speed and agility over an irregular terrain. In the cat family and the baboons, where the gait is digitigrade, the need for stability is greatest during plantar-flexion. It is mainly during this phase that fibular rotation occurs in these animals. On the other hand, in those primates and bears that exhibit a plantigrade type of gait the need for stability is greatest during dorsi-flexion. Experimental findings have confirmed that rotation of the fibula occurs during this phase. In the larger Pongidae and the bears the sides of the talus are divergent; this is presumably related to a frequent assumption in these animals of an upright posture which necessitates a wide foot through which the body weight can be distributed. It is probable that the os malleolare in the Artiodactyla likewise serves to stabilize the ankle joint against inversion and eversion strains, to which these animals are prone as a result of the cursorial characteristic of an elongation of the distal limb segment. A similar elongation is found in the Equidae; here the proximal portion of the fibula, articulating with the tibia at its upper end by means of a synovial joint, may be comparable in function to the slender fibula of saltatorial mammals. The movement permitted at the ankle is a hinge-like flexion and extension about an axis which is usually parallel to the ground and at right angles to the long axis of the limb. This is ideally adapted to a four-footed mode of progression, especially in cursorial mammals (Lull, 1904), where pure flexion and extension are required without the associated abduction and adduction which an axis not parallel to the ground would inevitably produce. These conclusions concerning the functional significance of the mobile and immobile types of fibula receive interesting support from the consideration of two highly specialized species in which the combination of tibio-fibular articulations appears to be unique. The fibula of the cheetah, Acinonyx jubatus, is long and slender and is firmly united to the tibia by fibrous tissue in the middle third of its shaft. The superior and inferior joints are synovial (Fig. 10). The fibula is sufficiently flexible to allow movement at both the extremities of the bone in spite of the fixation of the mid-portion of the shaft. This unusual combination can be correlated with the remarkable speed of this animal which probably exceeds 65 miles/hr. over short distances (Howell, 1944). The inferior articulation is characteristic of those animals adapted for progression over uneven surfaces, while the upper joint in combination with the mid-shaft syndesmosis is similar to that found in the hare, jerboa and tarsier, all of which are capable of rapid jumping and bounding movements. // The second animal of special interest is the coati, Nasua nasua, which possesses a slender fibula fused to the tibia at the upper end and articulating by a synovial joint at the lower, where Fig. 10. Cheetah, rotatory movement is permitted by virtue of the flexibility of the Aixnonyx jubatus. fibula. The upper end is typical of swimming and burrowing animals, while the lower end again resembles that found among animals adapted for uneven surfaces. This combination can probably be explained by reference to its reputed ability to dig burrows in the ground and also to hunt in trees. The rotatory mobility of the fibula in eutherian mammals 19 Representatives of the intermediate type of fibula are found in several mammalian orders. A small syndesmosis at the inferior articulation appears to be characteristic of those mammals which are not specialized in their mode of locomotion, that is to say those species showing no modification towards a cursorial, saltatorial, fossorial or aquatic mode of life, e.g. the great anteater, tapir and hippopotamus. The fibular mobility of this group, in which the upper joint is synovial, clearly depends on the extent of the syndesmosis at the lower end. Although no mobility has been detected experimentally in the dog and the pig, in which the syndesmosis is relatively large, the possibility of some movement cannot be excluded in those animals in which the syndesmosis is short. It is evident that within this group there are several species which exhibit some of the characteristics of the specialized groups. For example, the red fox, Vulpes vulpes, possesses an extensive tibio-fibular syndesmosis associated with a slender, flexible fibula which is functionally reminiscent of that found in the group capable of sudden, rapid limb movements. No rotation was detected experimentally in this species. On the other hand, the spotted hyaena, Crocuta crocuta, which shows a well- marked semilunar articular facet between the lower ends of the tibia and fibula in addition to a short syndesmosis, resembles in this respect those animals which are specially adapted for progression over uneven surfaces. Man shows a similar tendency-although the inferior tibio-fibular articulation frequently lacks a well- marked synovial recess, there is evidence that rotation of the fibula occurs (Bonnin, 1950). It seems probable that an articulation resembling the intermediate type has given rise, by adaptive radiation, to all the diverse forms of tibio-fibular relationship found in mammals with specialized modes of locomotion (Fig. 11). This view is in accordance with that of Carleton, who points out that in primitive vertebrates the tibia and fibula were not in contact at the lower end. It is likely, therefore, that before the development of a synovial joint inferiorly there must have been a phase where the two bones were united by fibrous tissue. This condition is illustrated in the present series by the intermediate type. Matthew (1937), on the other hand, maintains on palaeontological grounds that the type of articulation now found in large arboreal mammals represents the primitive placental form. It is widely accepted that the common ancestor of all placental mammals was a small arboreal insectivore resembling in size and mode of life the modern tree- shrew, which in common with other small arboreal forms belongs to the intermediate group. It can be inferred that these mammals have retained the ancestral tibio-fibular relationship, which then, as now, was presumably determined by functional needs. These small arboreal mammals, which can progress along the curved surface of a branch as if it were flat, are in contrast to the large arboreal forms in which a readily adaptable foot is a prerequisite; as described above, such adaptability necessitates the presence of a mobile fibula. Increase in size, therefore, may have been largely responsible for the development of an inferior synovial joint. The somewhat unspecialized tibio-fibular articulation of man may indicate an early divergence from the primitive stock at a stage when his arboreal ancestor was still of small size. 2-2 20 C. H. Barnett and J. R. Napier

SPECIALIZATION SPECIALIZAT ION FOR RAPID JUMPING FOR BURROWING MOVEMENTS AND SWIMMING

Cape golden mole Jerboa Desman Giant armadillo

Dog Seal

Pig Aard-vark

SPECIALI ZATION FOR UNEVEN SURFACES SPECIALIZATION FOR FAST RUNNING

Deer Cats G raffe Bears Most Primates

Tree-shrew Tapir Great anteater

Fig. 11. Scheme illustrating an adaptive radiation from a primitive type of tibio-fibular articulation towards four important specializations, with examples from different orders.

SUMMARY 1. The rotatory mobility of the fibula has been studied in the hind-limb bones of ninety-two representative eutherian mammals. In addition, twenty-two wet specimens have been dissected, and in many of these the relative movements of the fibula and tibia have been directly observed by means of indicator pins driven into the bones after they have been stripped of all muscles. Where there is fibular mobility, the principal movement is a lateral rotation occurring as the foot is passively dorsi- flexed from the plantar-flexed position. 2. An attempt is made to explain the mechanisms underlying fibular mobility among the mammals. The rotatory mobility of the fibula in eutherian mammals 21 3. On the basis of the anatomy of the tibio-fibular articulations it is possible to classify the majority of fibulae into three main types: the immobile fibula, fused to the tibia at one or both ends; the mobile fibula, articulating with the tibia by synovial joints at both ends; and the intermediate type, where the upper joint is synovial and the lower a syndesmosis. A few mammals show specializations of the fibula which exclude them from a classification of this nature. These are briefly discussed. 4. The immobile fibula is always associated with a talus of which the lateral articular surface is perpendicular to the axis of rotation at the ankle joint. There are two main varieties: (a) A thin, flexible fibula, fused or incorporated with the tibia at its lower end, and articulating with it at the upper end by means of a synovial joint. This variety is typical of limbs capable of sudden, rapid movements, as in saltatorial mammals. (b) A stout, inflexible fibula, fused with the tibia at its upper end and united with it at its lower end by bone or fibrous tissue. This variety is characteristic of fossorial and aquatic mammals. 5. The mobile fibula is restricted to certain members of the Carnivora and the Primates, and is associated with the ability to adapt the foot to uneven surfaces. The lateral articular surface of the talus is seldom perpendicular to the axis of rotation at the ankle joint. 6. The intermediate type of fibula includes those approximating most closely to a primitive pattern, as well as others showing some degree of modification towards the types adapted for specialized modes of progression. We are greatly indebted to Dr F. C. Fraser and Miss J. E. King of the Osteological section of the Zoology Department of the British Museum for the help they have given us throughout this investigation. Our thanks are due also to Prof. F. Wood Jones and Prof. D. V. Davies for the material they have placed at our disposal. We acknowledge with thanks the help of Miss F. Woolcombe who is responsible for the majority of the drawings and Mr A. L. Wooding and Mr J. B. Chanter for the photographic work.

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