Page:The American Cyclopædia (1879) Volume XI.djvu/84

 MAMMALIA where also are given sufficient details on the organs of sense, teeth, digestive system, and Lurv covering. The lower jaw consists of two pieces, and is alone movable; in man it is sus- ceptible ..f iii..t ion up and down, laterally, and from before backward; in the carnivora the first movement, in the ruminants the second, and in the rodents the third, is specially provided for 1'v the shape of the condyles and the form of the glenoid cavity. The limbs of mammals vary exceedingly in shape, according to the offices to be performed by them; we find the hand of man with its thumb opposable to the the four hands of the monkey, the 3 of the whale, the walking feet of the horse, the wing of the bat, the paw of the lion, the shovel of the mole, all constructed on the same type and modified from the same bony dements. The anterior limbs are always present, with a well developed scapular arch, usually kept in place by a clavicle; this last is present in man, monkeys, the insectivora, squirrels, and bats, but absent in cetaceans, the hoofed animals, and some edentates; in most carnivora and in some rodents it is imperfect-. lv developed; it corresponds to the furcular bone in birds, and the monotremata have in ail-lition the second or coracoid clavicle of birds. The shoulder blade is thin, flat, and more or less triangular, generally with a well marked spine; it is long and narrow in herbi- vora, and placed perpendicularly on the anterior an 1 lateral portion of the chest; in carnivora and rodents, requiring more freedom of motion, it is oblique, and so of course is the glenoid cavity; jockeys are well aware that an upright shoulder is the mark of a stumbling horse. The arm bone is nearly straight in man, much bent in the carnivora, long in monkeys and sloths, aii'l short in ruminants and cetaceans; it is con- 1 by a ball and socket joint with the scap- ula; below it articulates with the radius and ulna of the forearm by a hinge joint. The ulna is the longest in man and lies on the inside, and iv.vi ves the arm bone in a deep siginoid cavity; the radius is connected with the wrist, and turns with the movements of the hand, rolling around and upon the ulna; this independence of movement becomes less and less, acccording as the limbs are more used as instruments of ision; in the carnivora and rodents the two bones are distinct, but the rotation is very imperfect, and in the hoofed animals generally the two make a single bone; the radius seems to form the principal bone, the ulna being fre- quently, as in the horse and bats, very rudi- mentary. The wrist in man consists of eight bones in two rows, in other mammals varying from five to eleven; to these are attached the five parallel metacarpal bones in man, followed hy the five fingers, each having three joints, except the first or thumb, which has only two; ruminants the two metacarpals form the single cannon bone, sometimes with rudi- ry bones on the side, as the splint bones horse; most pachyderms have three metacarpals, the elephant having five. In ani- mals which walk on the ends of the toes, the metacarpus is so lengthened that it has been mistaken for the forearm, and supposed there- fore to be flexed in an opposite direction to that of man; but the lower part of the fore leg of a horse, for instance, is in reality the meta- carpus, and the part called the knee is the wrist -joint. The fingers vary from one to five; the third or middle finger is the most constant, and commonly the longest, and is the only one found in the horse; the thumb disappears first, then the little finger, and then the fourth finger; ruminants have the second and third, or fore and middle fingers. The hind limbs are more firmly connected to the trunk than the ante- rior; the supporting arch is the pelvis, com- posed of the ilium, ischium, and pubis on each side, the first joining the sacrum, the second forming the prominences upon which man sits, and the third uniting in front; in cetaceans there is only a rudiment of this bony arch, and the hind limbs are absent. The thigh bone, the longest in man, is in most other mammals relatively shorter; it is attached by a ball and socket joint to the pelvis, in man its axis being nearly that of the body, but in the lower mam- mals bending more and more forward until it forms an acute angle with the trunk. The tibia and fibula correspond to the radius and ulna of the forearm, and have the patella or knee-pan in front of the articulation with the thigh bone; these are coalesced in various ani- mals somewhat as are the radius and ulna; the tarsal bones correspond to the carpal, and are followed in the same manner by the metatarsus and toes. In the apes the great toe is opposable to the others, like the thumb, whence they are called quadrumana, four-handed; while man rests his whole foot, from the lieel to the toes, on the ground, other mammals walk chiefly on the toes; the horse stands on the tips of the middle fingers and toes, the heel being nearly as high up as the knee in man, the cat on the last two joints of several toes, and the bear on the metatarsus and toes; there is no animal, ex- cept man, that can be properly said to touch the ground with the entire foot; in the seals all the bones of the leg and foot may be recog- nized, but they are united by a membranous web into a kind of caudal fin. The bones of mam- mals have not the air cells found in birds, but are either solid or their cavities are filled with an oily matter called marrow; there are, how- ever, air cavities called sinuses, especially large in the frontal bone of ruminants, as in the ox and sheep, and greatly developed in the fron- tal region of the elephant; these communicate either with the nasal or auditory passages. While most mammals resemble man in the arrangement of the muscles, others approach birds and even fishes in this respect; as they are less active than birds, their muscles are less firm and the tendons less liable to ossify; they are generally fewer in number than in man, and their variations from the human type are