So

by side like the wires running from pole to pole along a telegraph line. Each of these neurones or nerve cells of the optic nerve has one end in the retina of the eye and the other in the brain (in the parts called the geniculate bodies). Again, if we could see exactly the structure of the brain itself, we should find it to consist of millions of similar neurones each resembling a bit of string frayed out at both ends and here and there along its course. also the nerves going out to the muscles are simply bundles of such neurones, each of which by itself is a threadlike connection between the cells of the spinal cord or brain and some muscle. The nervous system is simply the sum total of all these neurones, which form an almost infinitely complex system of connections between the sense organs and the muscles.

Fig. 5 gives a general idea of the essential features of a neurone, or nerve cell, by giving simplified drawings of several types of neurones.1 Figs. 6-16 may help to make real the idea that the brain and other components of the nervous system are essentially a vast assemblage of these string-like neurones. Figs. 6-11 are reproductions of drawings made with all possible fidelity to the actual facts; Figs. 12-16 reproduce actual photographs of very thin sections from the central nervous system, so stained as to show its composition. In examining them one must bear in mind that such sections will rarely if ever show the whole of any one neurone, but only cross views of parts of some, lengthwise views of parts of others, here a main string, there a frayed-out end. It is also the case

1 The student will of course bear in mind that the fraying out is not simply in one plane, that the neurone is not flat. Moreover, in the drawings the thickness of the string and its branches is overestimated relatively to their length. To get a true idea of these relative proportions in the longest neurones one would need to have a page many yards long or on a page of this size to represent the main string of the neurone as less than a thousandth of an inch thick.

that the method of staining used is such that not all of the neurones are stained. If they were, the whole picture would be a dense black mass, so closely are the stringlike nerve units packed together. The top of Fig. 15 gives some idea of this closeness of interweaving.

If the reader will think of a slice through the brain, such as appears to the naked eye as in Fig. 17 or Fig. 4, as being really of the appearance which a combination of hundreds of such drawings and photographs as Figs. 6-16 would make, he will have a true though crude conception of the general characteristics of the brain's structure. It is absolutely essential that the picture of it as a custardlike mass of gray and white stuff be replaced in the reader's mind by a picture of it as an aggregation of millions of thread-like neurones, each a perfectly definite unit by itself.

The drawings, I repeat, are in no sense unreal or simply general diagrams, but are actual reproductions of the things seen under the microscope. The photographs are of course absolute copies of actual cells as found in sections cut from the brain and stained. It is unfortunate that a picture of an entire neurone cannot be gotten by the camera, and that one sees therefore only a scrap of one neurone here and a scrap of another there. Figures drawn as well as photographs taken give a false idea of the length of the neurone. The longer ones do not appear for the very good reason that at the magnification. of say 190 diameters, a drawing of one of the longer neurones would have to be a tenth of a mile long.

1

2.

3.

4.

FIG. 6. A sketch showing elements of the structure of the brain cortex in mammals. These are drawn from actual specimens. Greatly magnified. After Edinger, 221, 152.

FIG. 7.

Kölliker, 652, 732.

RP

FIG. 8. A section through the brain cortex (a) (at the left) so stained as to show the thickened portions of the neurones and short pieces of their string-like processes, and (b) (at the right) so stained as to show only parts of the string-like processes. Imagine the left and right halves to fill the same space and the result will fairly represent the real condition. Much magnified. After Edinger, 220, 151.

FIG. 9.

A section of a part of the brain in a very early stage of its development. Much magnified. After Kölliker, 802, 814.

FIG. 10 (above) and FIG. II (below). Sections through the medulla oblongata or mylencephalon (the enlargement of the spinal cord where it joins the brain). These sections are from the brain of a young mouse, but the idea they give of the general structure of the brain is perfectly applicable to the human brain. After Van Gehuchten, II, 386, 597 and II, 392, 603. Both figures are due originally to Ramon y Cajal.