system, the nerve cells properly so called. This is demonstrated by the slide now before us, which shows us corresponding motor nerve cells of twelve different animals arranged in the order of their size—the elephant, the cow, the horse, man, the pig, the dog, the baboon, the cat, the rabbit, the rat, the mouse, and a small bat. You recognize immediately
Fig. 17. Motor Nerve Cells of Various Mammals, all from the cervical region of the spinal cord. The cells are represented, all uniformly magnified. After Irving Hardesty.
that there is a proportion between the size of these cells and the size of the respective species of animals. To a minor degree, but much less markedly, there is a difference in the caliber and length of the muscle fibers. But with these exceptions our statement is very nearly exactly true, that the difference in size of animals does not involve a difference in the size of their cells. For the purpose of the study of development, which we are to make in these lectures, this uni-
