Page:Popular Science Monthly Volume 43.djvu/394

380 as commonly termed, the region of the third ventricle, is a groove known as the sulcus of Munro, which runs from the opening which is termed the foramen of Munro, along the lateral wall of the ventricle, backward to the narrow continuation of the ventricle which has received the fanciful name of the aqueduct of Sylvius. This groove, the exact position of which I have thus indicated for the sake of possible anatomical readers, is the boundary between the dorsal and ventral zones. The superficial character of our previous knowledge of the brain is emphasized by the fact that the sulcus of Munro is usually not mentioned or figured in anatomical text-books, and yet we can say now that it is the most important landmark to be found in the part of the brain in which it occurs. It will suffice to give one other example: In the spinal cord the structure known by the name of the posterior fissure—a singular misnomer, since it is not a fissure—arises by the growing together of the two dorsal zones; a line drawn from the bottom of the so-called posterior fissure to the entrance of the posterior nerve roots would represent approximately the boundary between the dorsal and ventral zones. These two examples can, of course, be clear only to anatomists, but they demonstrate the permanency of the zonal divisions.

We have already learned that the fibers which arise from the nerve cells of the ganglia outside the nervous system proper enter the dorsal zone of His and there fork, the forks running longitudinally within the zone but in opposite directions. Gradually the number of fibers running in the zone increases until they form a fibrous tract of considerable size. The tract is originally situated next the outer surface of the nervous system; in the case of the spinal cord it remains permanently upon the outside, and therefore, as the nerve fibers ultimately become white in color, there is the so-called "white substance" covering the outer portion of the dorsal zone of the spinal cord, and it is this covering, which is known anatomically as the posterior columns, and which overlies all the medullary nerve cells that form i)art of the interior or "gray matter." In the brain also there enter several nerves, the ganglionic fibers which are distributed in precisely the same way as those just described—that is, they produce a superficial layer in the dorsal zone; they may be seen in this position during early stages in the part of the brain (medulla oblongata) adjoining the spinal cord. By secondary processes there follows a spreading of the nervous tissues over the outside of this white matter. We then have a white matter buried and isolated, but it remains, what it was primitively, the direct continuation of the superficial