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  Section: Anatomy of Vertebrate Animals » The Classification and Organization of the Mammalia
 
 
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The Edentata, or Bruta

 
     
 

In these Mammals the teeth are by no means always wanting, as the name of the group would seem to imply; but, when teeth are present, incisors are either altogether absent, or, at any rate, the median in cisors are wanting in both jaws. The teeth are always devoid of enamel, consisting merely of dentine and cement. As they grow for an indefinite period, they never form roots; and, so far as our knowledge at present extends, those which first appear are displaced by a second set only in some of the Armadillos. The ungual phalanges of the digits support long and strong claws.

There are mammae upon the thorax, and sometimes, in addition, on the integument of the abdomen; or, in the inguinal region.

The brain varies greatly, its hemispheres being sometimes quite smooth, with a very small corpus callosum and large anterior commissure; while, in other cases, the corpus callosum is much larger, and convolutions appear upon the surface of the brain.

The Edentata are divided into the Phytophaga, or vegetable-feeders, and the Entomophuga, or insect-eating forms. Leaves are the chief food of the former group, while the latter delight chiefly in ants, though some take, in addition, worms and carrion.

1. In the Phytophaga the long bones are without medullary cavities. The lateral part of the zygomatic arch sends down a remarkable vertical process. The acromial process of the scapula coalesces with the coracoid. In the carpus, the scaphoid and the trapezial bones anchylose and form one. The ischia become united with the anterior caudal vertebree, and these anchylose with the proper sacrals to form the long sacrum.

The ankle-joint has the character of a peg and socket, and the hind-foot is, more or less completely twisted, resting upon its outer edge, and not upon its sole.

Vascular canals connected with the pulp-cavity traverse the dentine of the teeth.

The Phytophaga are divisible into two groups, one existing, and the other extinct. The former consists of the Sloths, or Tardigrada; remarkable animals, which are confined to the great forests of South America, where they lead a purely arboreal life, suspended by their strong, hooklike, claws to the branches of the trees.

Their distinctive characters are these: The tail is short, and the limbs exceedingly long and slender, the anterior being longer than the posterior pair. In both the fore-and the hind-limbs the internal and the external digits are rudimentary, but the hind-foot always has the three middle toes completely developed; while, in the fore-foot, it sometimes happens that only two remain. The ungual phalanges are very long and hooked.

The zygomatic arch is incomplete posteriorly, not being united by bone with the squamosal. The cervical vertebrae in this remarkable group sometimes exceed, and sometimes fall short of, the number (seven) which is so characteristic of the Mammalia in general; some species of Sloths having nine, and others only six, vertebrte in the neck.

The pelvis is exceedingly spacious, and the acetabula are directed backward as well as outward. The femur is devoid of a ligamentum, teres. The distal end of the fibula sends inward a process which fits into a fossa situated upon the outer surface of the astragalus, giving rise to that kind of peg-and- socket ankle-joint which is peculiar to these animals.

A good deal of confusion prevails respecting the structure of the ankle-joint in the Sloths. Cuvier ("Ossemens fossiles," t. viii., p. 143) writes of the Ai, or three-toed Sloth:

"In the greater number of animals, the principal articulation of the astragalus connects it with the tibia, by means of a more or less loose ginglymus, which allows the foot to be bent on the leg. But here the principal and superior facet of the astragalus is a conical fossa, into which the pointed extremity of the fibula penetrates, like a pivot. (See PI. 208, Fig. 2a.) The inner edge of this fossa turns against a very small facet, which occupies only a third of the lower head of the tibia. The result of this arrangement is that the foot turns on the leg, like a weathercock on its support, but that it cannot be flexed. It further follows that the plane of the sole of the foot (Cuvier's words are: "II en resulte encore que le plan, le corps du pied, est presque vertical quand la jambe Test.") is almost vertical when the leg is so, and that the animal can only place the plantar surface of its foot on the ground by spreading out the leg so as to make it almost horizontal."

Meckle ("System der vergleichenden Anatomie," 2te Theil., 2te Abtheilung, p. 457.) has already justly remonstrated against Cuvier's assertion that only abduction and adduction are possible to the pes of the Ai, affirming that it is capable of flexion and extension, though only to a limited extent. A. Wagner follows Meckel, but Rapp ("Edentaten," p. 46) adopts Cuvier's statement in its fulness: "Extension and flexion of the foot cannot take place, but only abduction and adduction. "However, it is easy to demonstrate on the uninjured dead animal, or, still better, on the limb from which the muscles have been removed, while the ligaments have been left intact, that the pes of the three-toed Sloth is capable of extensive motion in three directions: first, in abduction and adduction; a movement in azimuth, when the leg is vertical; secondly, in flexion and extension; a more extensive movement in altitude, under the same circumstances; and, thirdly, in rotation upon its own axis, by means of which the sole can be moved through 90° from a position perpendicular to the axis of the leg to one parallel with it.

The anatomical arrangements upon which the execution of these movements depend are the following: The astragalus presents two facets to the bones of the leg, one of which (when the pes is in the position usual in other quadrupeds) looks inward and upward, while the other looks outward and upward. The former, convex from before backward, as well as from side to side, is by no means a mere rim, though it is not so wide as the other. It is the proper proximal surface of the astragalus, and articulates with the tibia. The other surface is excavated by a deep conical pit. Into this is received a correspondingly conical process of the distal end of the fibula, which is directed from above and without, downward and inward-not vertically, therefore, but very obliquely. Hence, even if the pivot fitted its socket quite accurately, there would still be abundant opportunity for flexion and extension, though the movement of the pes would be obliquely inward, as well as upward, in the former case; and obliquely outward, as well as downward, in the latter. But the socket fits the pivot loosely, and hence, as experiment demonstrates, the movement of the pes in flexion and extension is but very slightly oblique.

The true movement of abduction and adduction is so much less extensive than the movement in flexion and extension, because it is checked by the short and strong internal and external lateral ligaments of the ankle-joint.

With respect to the rotation of the foot on its own axis- it is to be observed, in the first place, that the calcaneum, cuboides, naviculare, the three cuneiformia, the three complete and the three rudimentary metatarsals, and the three basal phalanges of digits ii., iii., and iv., are anchylosed together into one bony mass; while, as in the manus, there is hardly any motion between the basal and the middle phalanges. Practically, in fact, the only bones of the pes which are movable upon one another are: 1. The distal phalanges, which have a movement of extension and flexion through 180° upon the middle phalanges. 2. The tarso-phalangeal synostosis above described is freely movable on the astragalus; and the joint is disposed in such a manner as to allow the sole of the foot to be rotated from the plantigrade position in which it is perpendicular to the axis of the leg, to the scansorial position, in which it lies parallel with the axis of the leg. It may be doubted, however, whether the former position can be given to the sole by the living animal. The tibialis anticus and the extensor hallucis longus are extremely strong muscles, and have no effcient antagonists; so that their tonic contraction must pull the navicular metatarsal tuberosity, into which they are inserted, as far upward as it will go, causing the tarso-phalangeal synostosis to rotate upon the astragalus, and thus obliging the sole of the foot to look inward.

In the two-toed Sloth, or Unau (Cholaepus), the general structure of the ankle-joint is the same, but the fossa of the astragalus looks almost directly outward, and the pivot of the fibula is more nearly horizontal, when the leg is vertical. The tibial facet of the astragalus looks directly upward. Hence, the movement of the pes is more exclusively one of flexion and extension than in the Ai. No anchylosis of the tarsal, metatarsal, and phalangeal bones occurs, but the rotation of the distal moiety of the tarsus upon the astragalus as much more complete and permanent than in the Ai. The calcaneum is twisted round under the astragalus, in such a manner that its proper external face becomes inferior, while the articular surface for the cuboid is not only below, but is partially internal to, the navicular facet of the astragalus. As a result of this position of the cuboid, the outer metatarsals, which it supports, are placed directly beneath the inner ones, and the pes rests absolutely upon its outer edge, the plane of the sole being vertical.

The Sloths, it thus appears, are naturally club-footed; but neither in the Ai, nor in the Unau, does this depend in any way on the structure of the ankle-joint. On the contrary, it results, in the Unau, from the manner in which the calcaneum and naviculare articulate with the astragalus; and, in the Ai, from the action of the muscles on the tarso-phalangeal synostosis. Neither in the Ai, nor the Unau, is there any thing to interfere with free flexion and extension of the pes.

The teeth are five in number on each side above, and four below, and become sharpened by mutual attrition into a chisellike form. The stomach is remarkably complex.

The Gravigrada are, for the most part, like the Sloths, South American forms, but they are entirely extinct; and while, in most respects, they resemble the Sloths, in others they present an approximation to Ant-eaters.

The jugal arch may be complete or incomplete. The articular surfaces of the dorsal vertebrae are sometimes complicated in a manner similar to that observed in the Ant-eaters. The tail is very long and strong. The limbs are short and subequal, while the fore-foot has the ulnar digit imperfect, as in the Ant-eaters. The fibula has no inward process, and the astragalus is consequently devoid of any fossa upon its outer surface. But another kind of peg-and-socket ankle-joint is produced by the interlocking of the surfaces of the tibia and of the astragalus.

The great extinct animals. Megatherium, Mylodon, Megalonyx, etc., the remains of which have been found almost wholly in later tertiary deposits of America, belong to this group.


 
     
 
 
     



     
 
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