Page:Popular Science Monthly Volume 71.djvu/373

Rh replaced by diversity in the composition of the various parts of the single cell. This figure shows us more clearly the principle of structure of a nerve cell, for here we have the central body of the cell composed of protoplasm with its nucleus in the middle and a small spot in the center of the nucleus, and these long branching processes running out in all directions which can take up nerve impulses from other similar or dissimilar cells, as the case may be, and carry them to the central body. To carry the message out there is typically but one process, which is different in appearance from the other processes which carry the impulses in. The latter are branching and are therefore called the tree-like or dendritic processes. Here is a single process like a long thread to carry the impulses away, and which

is called the axon of the nerve cell. In this case the modification of the shape of the cell has adapted it to the better performance of its functions. Notice also in these cells the enormous increase in the amount of protoplasm as compared with the nucleus. In the young cell of the rabbit germ, of which I showed you several illustrations a few moments ago, we had very little protoplasm for each nucleus, but here the protoplasm has many, many times the volume of the nucleus, and this is a relatively old cell.

Next let us look again at the figure of the striated muscle fiber, which you may recall from the second lecture, so that it will suffice if your attention is again directed to the oval nuclei, and to the lines stretching crosswise on the muscle giving it a "striated" appearance. You remember, doubtless, that such fibers are the ones which enable us to make voluntary motions. Originally each fiber was a set of