Page:A Text-book of Animal Physiology.djvu/58

 with those of higher animals. The condition of things in such an animal as Amœba may he compared to a civilized community in a very crude social condition. When each individual tries to perform every office for himself, he is at once carpenter, blacksmith, shoemaker, and much more, with the natural result that he is not efficient in any one direction. A community may be judged in regard to its degree of advancement by the amount of division of labor existing within it. Thus is it with the animal body. We find in such a creature as the fresh-water Hydra, consisting of two layers of cells forming a simple sac, a slight amount of advancement on Amœba. Its external surface no longer serves for inclosure of food, but it has the simplest form of mouth and tentacles. Each of the cells of the internal layer seems to act as a somewhat improved or specialized Amœba, while in those of the outer layer we mark a beginning of those functions which taken collectively give the higher animals information of the surrounding world.

Looking to the existing state of things in the universe, it is plain that an animal to attain to high ends must have powers of rapid locomotion, capacity to perceive what makes for its interest, and ability to utilize means to attain this when perceived. These considerations demand that an animal high in the scale of being should be provided with limbs sufficiently rigid to support its weight, moved by strong muscles, which must act in harmony. But this implies abundance of nutriment duly prepared and regularly conveyed to the bones and muscles. All this would be useless unless there was a controlling and energizing system capable both of being impressed and originating impressions. Such is found in the nerves and nerve-centers. Again, in order that this mechanism be kept in good running order, the waste of its own metabolism, which chokes and poisons, must be got rid of—hence the need of excretory apparatus. In order that the nervous system may get sufficient information of the world around, the surface of the body must be provided with special message-receiving offices in the form of modified nerve-endings. In short, it is seen that an animal as high in the scale as a mammal must have muscular, osseous (and connective), digestive, circulatory, excretory, and nervous tissues; and to these may be added certain forms of protective tissues, as hair, nails, etc.

Assuming that the student has at least some general knowledge of the structure of these various tissues, we propose to tell in a simple way the whole physiological story in brief.