walk. Lizards are light and agile, but progression is aided by a wriggling of the body. The locomotive organs of the mammalian quadrupeds are much more highly organized. The bones are more compact; the vertebral column is arched and yet elastic, between the shoulder and hip, and the limbs are placed vertically underneath the body. The bones of the fore limb are nearly in a line; but those of the hind limb, which is mainly used to project the body forward, are more or less inclined to one another, the angle being B A C FIG. 325.- Feet of Carnivores: A, Plantigrade (Bear); B, Pinnigrade (Seal); C, Digitigrade (Lion). most marked in animals of great speed, as the horse. Some walk on hoofs, as the ox (ungulate); some on the toes, as the cat (digitigrade); others on the sole, touching the ground with the heel, as the bear (plantigrade). In the pinnigrade seal, half of the fore limb is buried under the skin, and the hind limbs are turned backward to form a fin with the tail. The normal number of toes is five; but some may be wanting, so that we have onetoed animals (as horse), two-toed (as ox), three-toed (as rhinoceros), four-toed (as hippopotamus), and five-toed (as the elephant). The horse steps on what corresponds to the nail of the middle finger; and its swiftness is conditioned on the solidity of the extremities of the IV FIG. 326. - Feet of Hoofed Mammals: A, Elephant; B, Hippopotamus; C, Rhinoceros; D, Ox; E, Horse. a, astragalus; cl, calcaneum, or heel; s, naviculare; b, cuboides; ce, ci, cm, cuneiform bones; the numbers indicate the digits in use. limbs. Horses of the greatest speed have the shoulder joints directed at a considerable angle with the arm. The order in which the legs of quadrupeds succeed each other determines the various modes of progression, called the walk, trot, gallop, and leap. Many, as the horse, have all these movements; while some only leap, as the frog and kangaroo. In leaping animals, the hind limbs are extraordinarily developed. In many mammals, like the squirrel, cat, and dog, the fore legs are used for prehension as well as locomotion. Monkeys use all four, and also the tail, for locomotion and prehension, keeping a horizontal attitude; while the apes, half erect, as if they were half quadruped, half biped, go shambling along, touching the ground with the knuckles of one hand and then of the other. In descending the scale, from the most anthropoid ape to the true quadruped, we find the center of gravity placed increasingly higher up that is, farther forward. Birds and men are the only true bipeds, the former standing on their toes, the latter on the soles of the feet. Terrestial birds walk and run; while birds of flight usually hop. The ostrich can for a time outrun the Arabian horse; and the speed of the cassowary exceeds that of the swiftest greyhound. CHAPTER XXI* THE NERVOUS SYSTEM Nerve Tissue exists in the form of cells and fibers, the latter being prolongations of the bodies of the former. Where the cells predominate, nerve tissue is grayish. Such accumulations are called ganglia, or nerve centers, and these alone originate nervous force; the fibers are generally white, and arranged in bundles, called n η FIG. 329.-Nerve cell FIG. 330.- Diagram of Nervous Sys- nerves, which serve only as conductors. Most nerves contain two kinds of fibers, like in structure, but each having its distinct office: one carries impressions received from the external world to the gray centers, and hence is called an afferent, or sensory, nerve; the other conducts an influence generated in the center to the muscles, in obedience to which they * See Appendix. contract, and hence it is called an efferent, or motor, nerve. Thus, when the finger is pricked with a pin, afferent nerve fibers convey the impression to the center, the spinal cord, which immediately transmits an order by efferent fibers to the muscles of the hand to contract. If the former are cut, sensation is lost, but voluntary motion remains; if the latter are cut, the animal loses all control over the muscles, although sensibility is perfect; if both are cut, the animal is said to be paralyzed in the parts which these nerves control. The nerve fibers are connected with nerve cells in the central organs, and at the outer ends are connected with the muscular fibers, or with various sensory end organs in the skin or other parts of the body. The nature of nerve force is not known. As to the velocity of a nervous impulse, we know it is far less than that of electricity or light, and that it is more rapid in warm-blooded than in cold-blooded animals, being faster in man than in the frog. In the latter it averages about 85 feet per second, the former over 100 feet. FIG. 331.- The very lowest animals, like the amoeba and Infusoria, have no nerves, although their protoplasm has a general sensibility. The hydra has certain cells which are, perhaps, partly nervous and partly muscular in function. The jellyfish has a nervous system, consisting of a network of threads and ganglia scattered all over its disk. We should look for a definite system of ganglia and nerves only in those animals which possess |