The fore-arm is succeeded by the small bones which compose the wrist or "carpus." These are eight in number in Man, but vary in different Mammals from five to eleven.
The metacarpus in Man and in most Mammals consists of five cylindrical bones, articulating proximally with the carpus, and distally with the phalanges of the fingers. The most remarkable modification of this normal state of things occurs in the Ruminants and in the Horse. In the Ruminants, in which the foot is cleft, and consists of two perfect toes only, there are two metacarpal bones in the embryo; but these are anchylosed together in the adult, and form a single mass which is known as the "canon-bone" (fig. 354, c). In the Horse, in which the foot consists of no more than a single digit, there is only a single metacarpal bone, on each side of which are two little bony spines - the so-called "splint-bones" - which are attached superiorly to the carpus, and are to be regarded as rudimentary metacarpals. In most of the other Ungulates there are at least three metacarpals, and in the elephants there are five.
The normal number of digits is five, but they vary from one to five. The middle finger is the longest, and most persistent of the digits of the fore-limb; and in the Horse it is the only one which is left (fig. 355). The thumb is very frequently absent. In the Ruminants there are only two fingers which are functionally useful, these carrying the hoofs. In most Ruminants, however, there are two rudimentary and functionally useless digits in addition.
Normally each digit has three phalanges, except the thumb, which has only two. In the Whales and Dolphins (Cetacea), in which the anterior limbs form swimming-paddles, very like those of the Ichthyosaurus and Plesiosaurus, the phalanges are considerably increased in number as they are in those Reptiles. In all the Mammalia, too, except the Cetacea, it is the rule that the terminal phalanx in each digit should carry a nail, claw, or hoof.
Fig. 354. - A, Fore-leg of Ox (Bos taurus). B, Hind-leg of Stag (Cervus elaphus). ca Carpus; ta Tarsus; c "Canon-bone," composed of the united metacarpals or metatarsals of the 3d and 4th digits.
Fig. 355. - Fore-leg of Horse. ca Carpus; m Metacarpal of the third digit; s "Splint-bone," or rudimentary metacarpal ; 1, First phalanx or " great pastern; " 2, Second phalanx or " small pastern ;" 3, Third phalanx or " coffin-bone."
The power of opposing the thumb to the other digits of the hand is found only in Man, and in a considerable number of the Quadrumana, but never so perfectly developed as in Man. In Man only does this power attain its full perfection, and it constitutes one of the most striking of the merely anatomical peculiarities by which Man is separated from the Monkeys. As, however, this feature is purely adaptive, and is really to be regarded as of extremely small physiological value, we ought to learn from this that the difference between Man and the Quadrumana is to be sought in the mental powers of each, and not in any merely structural character.
Whilst the anterior limbs are never absent in any Mammal, the posterior limbs are occasionally wholly wanting, as in the Cetacea and Sirenia (in Halitherium, an extinct Sirenian, a rudimentary femur exists). Generally speaking, however, the posterior limbs are present, and the pelvic arch has much the same structure as in Man. The two halves of the pelvis - the ossa innominata - consist each of three pieces in the embryo - viz., the ilium, ischium, and pubes - which meet to form the cup-shaped cavity known as the "acetabulum," with which the head of the thigh-bone articulates. In the adult Mammal these three bones are anchylosed together, and the two ossa innominata unite in front by means of a symphysis pubis, constituted either by a cartilaginous union (synchondrosis), or by merely ligamentous attachment. In some Mammals, however, such as the Mole, and many of the Bats, the pubic bones remain disunited during life. As a rule, also, the ossa innominata are firmly united with the vertebral column. In the Cetaceans, in which the hind-limbs are wanting, and there is no sacrum, the innominate bones are rudimentary, and are not attached in any way to the spine.
The only other bones which are ever connected with the pelvis are two small bones which are directed upwards from the brim of the pelvic cavity in Marsupials and Monotremes. These are the so-called "marsupial bones," regarded generally as not forming parts of the skeleton properly so called, but as being ossifications of the internal tendons of the "external oblique" muscles of the abdomen (fig. 360).
In those Mammals which possess hind-limbs, the normal composition of the member is of the following parts : 1. A thigh-bone or femur; 2. Two bones forming the shank, and known as the tibia and fibula; 3. A number of small bones constituting the ankle or tarsus; 4. The "root" of the foot, made up of the "metatarsus;" 5. The phalanges of the toes (see fig. 243).
The thigh-bone or femur articulates with the pelvis, usually at a very open angle. In Man it is distinguished by being the longest bone of the body, and by having the axis of its shaft nearly parallel to that of the vertebral column. In most Mammals the femur is relatively shorter, and the axis of its shaft deviates considerably from that of the spine, being sometimes at right angles, or even at an acute angle.
Of the bones of the leg proper (crus) the tibia corresponds to the radius in the fore-limb, as shown by its carrying the tarsus; and the fibula is the representative of the ulna. The articulation between the tibia and fibula on the one hand, and the femur on the other, constitutes the "knee-joint," which is usually defended in front by the "knee-pan" or patella, a large sesamoid bone developed in the tendons of the great extensor muscles of the thigh. The patella is of small size in the Car-nivora, but does not appear to be wanting in any except in some of the Marsupials. In many cases the tibia and fibula are anchylosed towards their distal extremities. In the Horse the fibula has much the same character as in Birds, being a long splint-like bone which only extends about half-way down the tibia. In the Ruminants the reverse of this obtains, the upper half of the fibula being absent, and only the lower half present.