860 MAMMALIA [SKELETON. paddle, being covered by continuous integument, which conceals all trace of division into separate digits, and without sign of nails or claws. In the Sloths the manus is long and very narrow, habitually curved, and terminat ing in two or three pointed carved claws in close apposition with each other, incapable, in fact, of being divaricated, so that it is reduced to the condition of a hook, by which the animal suspends itself to the boughs of the trees among which it lives. These are only examples of the endless modifications to which the distal extremity of the limb is subjected in adaptation to the various purposes to which it is applied. Posterior The posterior limb is constructed upon a plan very limb. similar to that of the anterior extremity. It consists of a pelvic girdle and three segments belonging to the limb proper, viz., the thigh, the leg, and the foot or pes. Pelvic The pelvic girdle is present in some form in all mammals, girdle, though in the Cetacea and the Sirenia it is in an exceed ingly rudimentary condition. In all mammals except those belonging to the two orders just named, each lateral half of the pelvic girdle consists essentially, like the corre sponding part of the anterior limb, of a flattened rod of bone crossing the long axis of the trunk, having an upper or dorsal and a lower or ventral end. The upper end diverges from that of the opposite side, but the lower end approaches, and, in most cases, meets it, forming a symphysis, without the intervention of any bone corre sponding to the sternum. The pelvic girdle differs from the shoulder girdle in being firmly articulated to the vertebral column, thus giving greater power to the hinder limb in its function of supporting and propelling the body. Like the shoulder girdle, it bears on its outer side, near thn middle, a cup-shaped articular cavity (" acetabulum "), into which the proximal end of the first bone of the limb proper is received. Each lateral half of the girdle is called the " os innominatum," and consists originally of three bones which unite at the acetabulum. The " ilium " or upper bone is that which articulates with the sacral vertebrae. Of the two lower bones the anterior or "pubis" unites with its fellow of the other side at the symphysis ; the posterior is the " ischium." These two form two bars of bone, united above and below, but leaving a space between them in the middle, filled only by membrane, and called the " thyroid " or " obturator " foramen. The whole circle of bone formed by the two innominate bones and the sacrum is called the pelvis. In the Monotremata and Marsupialia, a pair of thin, flat, elongated bones called epipubic or marsupial bones are attached to the fore part of the pubis, and project forward into the muscular wall of the abdomen. Thigh The first segment of the limb proper has one bone, the and leg. femur, corresponding with the humerus of the upper limb. The second segment has two bones, the tibia and fibula, corresponding with the radius and ulna. These bones always lie in their primitive unmodified position, parallel to each other, the tibia on the preaxial and the fibula on the postaxial side, and are never either permanently crossed or capable of any considerable amount of rotation, as in the corresponding bones of the fore limb. In the ordinary walking position the tibia is internal, and the fibula external. In many mammals the fibula is in a more or hss rudimentary condition, and it often ankyloses with the tibia at one or both extremities. The patella or "knee cap" is found in an ossified condition in all mammals, with the exception of some of the Marsupialia. It is a large sesamoid bone developed in the tendon of the extensor muscles of the thigh, where the tendon passes over the front of the knee-joint, to which it serves as a protection. There arc frequently smaller ossicles, one or two in number, situated behind the femoral condyles, called " fabell*." The terminal segment of the hind limb is the foot or pes. Foot. Its skeleton presents in many particulars a close resemblance to that of the manus, being divisible into three parts : (1) a group of short, more or less rounded or square-shaped bones, constituting the tarsus ; (2) a series of long bones placed side by side, forming the metatarsus ; and (3) the phalanges of the digits or toes. The bones of the tarsus of many of the lower Vertebrata closely resemble both in number and arrangement those of the carpus, as shown in fig. 11. They have been described in their most generalized condition by Gegenbaur under the names expressed in the first column of the following table. The names in the second column are those by which they are most generally known to English anatomists, while in the third column some synonyms occasionally employed are added TiUale Intermedium Fibulare Centrale Tarsale 1 Tarsale 2 Tarsale 3 Tarsale 4 Tarsale 5 = Astragalus = Calcaneum = Navicular = Internal cuneiform = Middle cuneiform = Talus. = Os cakis. = Scaphoidcurn. = Entocuncifonnc. = Mesocunciformv. = External cuneiform = Ectocundforme. = Cuboid. The bones of the tarsus of mammals present fewer diversities of number and arrangement than those of the carpus. The proximal row (see fig. 12) always consists of two bones, the astragalus (a, which probably represents the coalesced scaphoid and lunar of the hand) and the calcaneum (c). The former is placed more to the dorsal side of the foot than the latter, and almost exclusively furnishes the tarsal part of the tibio-tarsal or ankle-joint. The calcaneum, placed more to the ventral or "plantar" side of the foot, is elongated backwards to form a more or less prominent tuberosity, the "tuber calcis," to which the tendon of the great extensor muscles of the foot is attached. The navicular bone (n) is inter posed between the proximal and distal row on the inner or tibial side of the foot, but on the outer side the bones of the two rows come into contact. The distal row, when complete, consists of four bones, which, beginning on the inner side, are the three cuneiform bones, internal (c 1 ), middle (c 2 ), and external (c 3 ), FIG. 12. nones of the of the navicular, and the cuboid (cb), articulated with the calca neum. Of these the middle cuneiform is usually the smallest in animals in which all five digits are developed ; but when tho hallux is wanting the internal cuneiform may be rudiment ary or altogether absent. The three cuneiform bones sup port respectively the first, second, and third metatarsals, and the cuboid supports the fourth and fifth ; they thus exactly correspond with the four bones of the distal row of the carpus. In addition to these constant tarsal bones, there may be supplemental or sesamoid bones : one situated near the middle of the tibial side of the tarsus, largely developed in many Carnivora and Rodcntia ; another, less frequent, on the fibular side ; and a third, often developed in the tendons articulated to the di.stal surface Fl ; 12. -nones of Human loot. T, tarsus; J/, metatarsus ; Ph, phalanges ; <. calcaneum ; a, astragalus ; cb. cuboid ; n, navicular ; c 1 , in ternal cuneiform; c 2 . middle cuneiform ; c 3 , external cunei form. The digits are indicated by Roman numerals, counting from the tibial to the flbular
side.