LAW OF SIMPLIFICATION OF FEET.

243

beginning of the radiating multiplication of its parts. The first segment is the seat of those modifications which have obtained for it the special name of "humerus," 53; the two divisions of the next segment of the appendage are called "ulna," 53, and "radius," 54; the remainder of the limb is called "manus," or hand; 56 is the gristly carpus, and the two bony divisions are the digits or fingers, 57. The segment supporting the hind-limbs retains most

h52 55

Fig. 34.
с

62

of its typical character in the subterranean reptile called the Proteus; one sees, c. g., in Fig. 34, that the centrum has coalesced with the neurapophyses, n, and neural spine, ns, forming the neural arch from which the diapophyses, d, are developed: the more expanded hæmal arch consists of the pleur. apophyses, pl, and the hæmapophyses, h; the former is called the "ilium," 62, the latter the "ischium," 63; and, as the hæmapophyses of another segment are usually added to the scapular arch, when they receive the name of "clavicles," so also the hæmapophyses of a contiguous segment are usually added to the pelvic arch, when they are called "pubic bones."

PROTEUS.

68 69

The pelvic diverging appendage, a a, has advanced to the same stage of complexity in the proteus, as the scapular one in the amphiuma; the first ossified segment is called “femur,” 65; the divisions of the next segment are respectively termed “tibia,” 66, and “fibula,” 67; the first set of short bones in the "pes," or foot, are called “tarsals," 68; those of the two toes are called "metatarsals" and "phalanges," 69. The tarsal bones, from the degree of constancy of their forms and relative positions have received distinct names. Fig. 35. Fig. 36. In Fig. 35 of the bones of the hind foot in the horse, a marks the "astragalus," cl, the "calcaneum," or heel-bone, the prominent part of which forms the "hock;"s is the "scaphoides," or naviculare, b, the "cuboid," ce, 66 .66 ectocuneiform," and cm, the meso

Fig. 37.

Fig. 38.

Fig. 39.

cuneiform." Now, the ectocuneiform in all mammalia supports the third or middle

244

LAW OF SIMPLIFICATION OF FEET.

of the five toes when they are all present, the mesocuneiform supports the second toe, and the cuboides the fourth and fifth. We see, therefore, in the horse, that the very large bone articulated to the ectocuneiform, ce, is the metatarsal of the third toe, to which are articulated the three phalanges of the same toe, iii, the last phalanx being expanded to sustain the hoof. The small bone called "splint-bone," by veterinarians, articulated to the "mesocuneiform," is the stunted metatarsal of the second toe, ii; the outer "splint-bone," articulated to the "cuboides," is the similarly stunted metatarsal of the fourth toe, iv.

In the foot of the ox (Fig. 36), the cuboides, b, presents a marked increase of size, equalling the ectocuneiform, ce, which is proportionally diminished. The single bone, called "cannon-bone," which articulates with both these carpal bones, does not answer to the single "cannon-bone" in the horse, but to the metatarsals of both the third and the fourth digits; it is accordingly found to consist of those two distinct bones in the fatal ruminant, and there are a few species in which that distinction is retained. Marks of the primitive division are always perceptible, especially at its lower end, where there are two distinct joints or condyles, for the phalanges of the third, iii, and fourth, iv, toes. In the horse the rudiments of the two stunted toes were their upper ends or metatarsal bones; in the ox they consist of their lower ends or phalanges; these form the "spurious hoofs," and are parts of the second, ii, and fifth, v, toes (Fig. 36). The rhinoceros more closely resembles the horse in the bony structure of its hind foot (Fig. 37); the ectocuneiform is still the largest of the three lowest tarsal bones, although the mesocuneiform, cm, and the cuboids, b, have gained increased dimensions in accordance with the completely developed toes which they support; these toes we therefore recognise as being answerable to the rudiments of the second, ii, and fourth, iv, in the horse, the principal toe being still the third, i. The hippopotamus (Fig. 38) chiefly differs from the ox, as the rhinoceros differs from the horse, viz., by manifesting the two toes fully developed, which were rudimental in the more simple foot; the cuboides, b, being proportionally extended to support the fifth toe, v, as well as the fourth, iv; the second toe, ii, articulates, as usual, with a distinct tarsal bone. In the elephant (Fig. 39), where a fifth digit is added, answering to our first or great toe, I, there is also a distinct carpal bone, called the "entocuneiform," ci, and the tarsus presents, as in other pentadactyle mammals, all the bones which are seen in the human tarsus, viz., the astragalus, a, the calcaneum, c, the scaphoides, s, the entocuneiform, ci, the mesocuneiform, cm, the ectocuneiform, cc, and the cuboides, b.

The course of the simplification of the pentadactyle foot or hand is first a diminution and removal of the innermost digit, i; next of the outermost, v; then of the second, i; and lastly of the fourth, iv; the third or middle toe, iii, being the most constant and important of the five toes. The same law or progress of simplification prevails in the fore-foot or hand. The thumb is the first to disappear, then the little finger, and the middle finger is the most constant, and forms the single-hoofed fore-foot of the horse.

The scapula, 51, in the fore-limb repeats or answers to the ilium, 62, in the hind limb; the coracoid, 52, to the ischium, 63; the clavicle, 58, to the pubis, 64; the humerus, 53, to the femur, 65; the radius, 55, to the tibia, 66; the ulna, 54, to the fibula, 67; the carpus, 56, repeats the tarsus, 68; and the metacarpus and phalanges of the fore-foot repeat the metatarsus and phalanges of the hind foot; they are technically called "serial homologues," or "homotypes," and each bone in the carpus can be shown to have its homotype in the tarsus.-See "Archetype of the Vertebrate Skeleton," p. 167.

SKELETON OF THE SLOTH.

245

Skeleton of the Sloth.-The transition from the quadrupeds with hoofs to those with claws seems in the present series to be aorupt; but it was made gradual by a group of animals, now extinct, which combined hoofs and claws in the same foot. Some of the outer toes, at least, were stunted and buried in a thick callosity, for the ordinary purpose of walking, whilst the other toes were provided with very long and strong claws for uprooting or tearing off the branches of trees. These singular beasts were of great bulk, and appear to have been peculiar to America. As restored by anatomical science, they have received the names of Megatherium, Megalonyx, Mylodon, &c. They were huge terrestrial sloths; the present remnants of the family consist of very few species enabled by their restricted bulk to climb the trees in quest of their leafy food, and peculiarly organized for arboreal life. The toes, which were modified in their huge predecessors, to tread the ground, are reduced to rudiments, or are undeveloped; and those only are retained which support the claws, now rendered by their length and curvature admirable instruments for clinging to the branches. The whole structure of the hind and fore limbs is modified to give full effect to these instruments as movers and suspenders of the body in the bosky retreats for which the sloths are destined; and, in the same degree, the power of the limbs to support and carry the animal along the bare ground is abrogated. Accordingly, when a sloth is placed on level ground, it presents the aspect of the most helpless and crippled of creatures. It is less able to raise its trunk above its limbs than the seal, and can only progress by availing itself of some inequality in the soil offering a holdfast to its claws, and enabling it to drag

itself along. But to judge of the creative dispensations towards such an animal by observation of it, or reports of its procedure, under these unnatural circumstances.

246

APTITUDES OF THE SLOTH.

would be as reasonable as a speculation on the natural powers of a tailor suddenly transferred from his shop-board to the rigging of a ship under weigh, or of a thoroughbred seaman mounted for the first time on a full-blood horse at Ascot. Rouse the prostrate sloth, and let it hook on to the lower bough of a tree, and the comparative agility with which it mounts to the topmost branches will surprise the spectator. In its native South American woods its agility is still more remarkable, when the trees are agitated by a storm. At that time the instinct of the sloth teaches it that the migration from tree to tree will be most facilitated. Swinging to and fro, back downwards, as is its habitual position, at the end of a branch just strong enough to support the animal, it takes advantage of the first branch of the adjoining tree that may be swayed by the blast within its reach, and stretching out its fore-limb, it hooks itself on, and at once transfers itself to what is equivalent to a fresh pasture. The story of the sloth voluntarily dropping to the ground, and crawling under pressure of starvation to another tree, is one of the fabulous excrescences of a credulous and gossipping zoology.

In the sloth, accordingly (Fig. 40), the fore-limbs are much elongated, and that less at the expense of the hand than of the arm and fore-arm. The humerus, 53, is of unwonted length-is slender and straight; the radius, 55, and ulna, 54, are of similar proportions—the former straight, the latter so bent as to leave a wide interosseous space. Now, moreover, these bones, instead of being firmly united as one bone, are so articulated with each other as to permit a reciprocal rotatory movement, chiefly performed, however, by the radius; and since to this bone the carpal segment of the hand is mainly articulated, that prehensile member can be turned prone or supine, as in the human arm and hand. Six bones are preserved in the carpus of three-toed sloth (Bradypus tridactylus), answering to those called "lunare," "cuneiforme," "unciforme," and "pisiforme," also to the "scaphoides and trapezium" united, and to the "trapezoides and magnum" united. The scapho-trapezium is characteristic of the sloth-tribe, and is found in the extinct as well as existing species. The articulation of the carpus with the radius, and with the metacarpus, is such as to turn the palm of the long hand inwards, and bring its outer edge to the ground. The three fully-developed metacarpals are confluent at their base, which is also anchylosed to the rudiments of the first and fifth metacarpals; the proximal phalanges of the digits answering to ii, iii, and iv, are confluent with their metacarpals, and those digits appear therefore to have only two joints. The last phalanx is remarkably modified for the attachment of the very long and strong claw. With regard to the blade-bone of the sloth, 51, it is much broader in proportion to its length than in the swift cloven-footed herbivores; the spinous process is unusually short; the acromion is of moderate length, and unexpanded at its extremity; the supraspinal fossa is the broadest, and has a perforation instead of the usual "supraspinal" notch. There is a short clavicular bone attached to the acromion, but not attaining to the sternum.

The iliac bones, 62, repeat the modifications of their homotypes the scapulæ, and are of unusual breadth as compared with those of other quadrupeds; they soon become anchylosed to the broad sacrum, S; the ischia, 63, and pubes, 64, are long and slender, and circumscribe unusually large" thyroid" and " ischial” foramina, the latter being completed by the coalescence of the tuberosities of the ischia with the transverse processes of the last two sacral vertebre. The head of the femur, 65, has no impression of a ligamentum teres. The patella, 66', is ossified; there is a fabella behind the external condyle. The tibia, 66, and fibula, 67, are bent in opposite directions, intercepting a very wide interosseous space. The anchylosis of their two extremities, which has been found in older speci

:

mens, has not taken place here. The inner malleolus projects backwards and supports a grooved process. The outer malleolus projects downwards, and fits like a pivot into a socket in the astragalus, turning the sole of the foot inwards-a position like that of the hand-best adapted for grasping boughs. The calcaneum, 68, is remarkably long and compressed. The scaphoid, cuboid, and cuneiform bones have become confluent with each other and the metatarsals, of which the first and fifth exist only in rudiment. The other three have likewise coalesced with the proximal phalanges of the toes which they support these toes answer to the second, third, and fourth, in the human foot. The short and small head of the sloth is supported on a long and flexible neck presenting the very unusual character in the Mammalian class of nine vertebræ, C-the superadded two, however, appearing to have been impressed from the dorsal series D by their short, pointed, and usually moveable ribs. The head and mouth can be turned round every part of a branch in quest of the leafy food by this mechanism of the neck. As the trunk is commonly suspended from the limbs with the back downwards, the muscles destined for the movements of the back and support of the head are feebly developed, and the vertebral processes for their attachment are proportionally short. The spines of the neck-vertebræ are of more equal length than in most mammals— that of the dentata being little larger than the rest: the spines gradually subside in the posterior dorsals, and become obsolete in the lumbar vertebræ. The first pair of fullydeveloped ribs, marking the beginning of the true "dorsal" series of vertebræ, are anchylosed to the breast-bone, which consists of eight ossicles. In the two-toed sloth, however, which has twenty-three dorsal vertebræ, there are as many as seventeen subcubical sternal bones in one long row, with their angles truncated for the terminal articulations of the sternal ribs, which are ossified.

The skull of the sloth is chiefly remarkable for the size, shape, and connections of the malar bone, which is freely suspended by its anterior attachment to the maxillary and frontal, and bifurcates behind; one division extending downwards, outside the lower jaw, the other ascending above the free termination of the zygomatic process of the squamosal. The premaxillary bone is single and edentulous, being represented only by its palatal portion completing the maxillary arch, but not sending any processes upwards to the nasals.

The skull in the toothless ant-eater chiefly forms a long, slender, slightly-bent bony sheath for its still longer and more slender tongue, the main instrument for obtaining its insect food. The mouth in the living animal is a small orifice at the end of the tubular muzzle, just big enough to let the vermiform tongue glide easily in and out. The fore-limbs are remarkable for the great size and strength of the claw developed from the middle digit: this is the instrument by which the ant-eater mainly effects the breach in the walls of the termite fortresses, which it habitually besieges in order to prey upon their inhabitants and constructors. As in the sloths, both fore and hind feet have an inclination inwards, whereby the sharp ends of the long claws are prevented from being worn by that constant application to the ground which must have resulted from the ordinary position of the foot. The trunk-vertebræ of the ant-eater are chiefly remarkable for the number of accessory joints by which they are articulated together. This complex structure is also met with in the armadillos, in which the anterior zygapophyses of the dorsal vertebræ send processes-the metapophyses (Fig. 2, p. 165), m, m-upwards, outwards, and forwards, which processes, progressively increasing in the hinder vertebræ, attain, in the lumbar region, a length equal to that of the spinous processes, ns, and have the same relation to them, in the support of the