STRUCTURAL PLAN OF THE HUMAN BRAIN 373 



medullary and the ganglionic respectively. Each part has its 

 special typical cells and nerve fibers. It is further probable that 

 there is still a third class of nerve fibers — namely, those connected 

 with the sensory apparatus of the special sense cells. 2. There 

 are three sets of nerve roots — namely, the true dorsal roots, which 

 are formed solely by ganglionic nerve fibers ; and the lateral and 

 the ventral roots, which are formed solely of medullary nerve 

 fibers; the lateral roots have been hitherto generally confused 

 with the dorsal roots ; they have been traced heretofore only in 

 the brain and in the cervical nerves, but I consider it more than 

 possible that the posterior roots of the spinal nerves will be found 

 to represent both dorsal and lateral roots. 3. Nerve fibers grow 

 out from a cell and the end of each fiber branches ; but, so far as 

 observed, none of the branches become materially continuous, 

 either with other nerves or nerve cells or with any other cells or 

 other protoplasmatic structures. 4. The entire brain and spinal 

 cord is divided into four principal longitudinal divisions, which I 

 have named after their discoverer the zones of His. The zones 

 are in pairs — that is to say, on each side there is a dorsal (i. e., in 

 the spinal cord " posterior ") and a ventral (i. e., in the spinal cord 

 " anterior ") zone. These zones are of fundamental importance, 

 because all the fibers which belong to the ganglionic portion of 

 the nervous system ramify in the dorsal zone, while all the fibers 

 belonging to the medullary portion leave the spinal cord (or 

 brain) through the ventral zone. Both zones persist throughout 

 life, and preserve their fundamental relations to the two kinds of 

 nerve fibers. 



Let us now attempt to acquire fuller and more exact concep- 

 tions in regard to the four discoveries above enumerated. We 

 may hope to do this without entering into technical details and 

 with the use only of terms readily understood. At the same time 

 we shall learn wherein the significance of the four discoveries lies. 



The First Discovery. — The division of the nervous system 

 into a medullary portion and a ganglionic portion has to be ex- 

 plained. The division has long been a familiar fact to anatomists, 

 but its true character and fundamental significance have been 

 known a short time only, because it is owing to very recent em- 

 bryological discoveries that the independent development of the 

 ganglionic portion has been elucidated. The existence of the gan- 

 glia has long been known, but their development independently 

 of the rest of the nervous system is a new conception. Their in- 

 dependence is, of course, not absolute but relative, for every part 

 of the body develops in intimate relations with, and in depend- 

 ence upon, the neighboring parts. 



By the medullary portion we understand the brain proper 

 plus the spinal cord or marrow and the nerve fibers, which grow 



