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 conceptions 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 explained. 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 embryological discoveries that the independent development of the ganglionic portion has been elucidated. The existence of the ganglia has long been known, but their development independently of the rest of the nervous system is a new conception. Their independence is, of course, not absolute but relative, for every part of the body develops in intimate relations with, and in dependence 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
