380 THE POPULAR SCIENCE MONTHLY. 



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 ventri- 

 cle which has received the fanciful name of the aqueduct of Syl- 

 vius. This groove, the exact position of which I have thus indi- 

 cated 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 longitu- 

 dinally 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 there- 

 fore, 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 over- 

 lies all the medullary nerve cells that form part 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 spread- 

 ing 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 



* Including the postero-lateral columns, the columns of Burdach, and perhaps also the 

 columns of Gol. 



