260 ANATOMY OF THE CBNTKAL NEEVOUS SYSTEM. 



now better understood. First of all^ there are the fibers from the parietal 

 region to the ventral nucleus. They must contain, as I have previously 

 stated, that portion of the sensory fiber-system which passes to the cortex 

 from this nucleus, where a part of the fillet terminates. Then we know that 

 the greatest part of the fibers converging into the posterior divisions of the 

 thalamus, especially into the pulvinar and the corpus geniculatum laterale, 

 arises from the occipital lobe, and represents the secondary pathway from 

 the primary optic terminals to the cortex. 



The inner side of the thalamus is separated from the ventricle by a 

 uniform layer of gray matter. This is called the central gray matter of the 

 middle (third) ventricle, and consists of a tissue rich in cells and traversed 

 in all directions by numerous fine, medullated nerve-fibers. 



Scliiit', who has made this gray matter a subject of especial study in man, 

 found that it contains afferent tracts from almost all of the ganglia surrounding the 

 third ventricle, and, Avhat is particularly interesting, that it degenerates like the 

 fibers of the cerebral cortex in progressive paralyses. A tract of fine, medullated 

 fibers, which may be traced in the gray matter from the third ventricle down as far 

 as the nuclei of the hypoglossus, has been named by him the dorsal longitudinal 

 bundle of the central gray matter. It is, for the most part, especially well defined, 

 and constantly lies close under the epithelium of the ventricle. 



In the median line of the brain the central gray matter forms the floor 

 of the ventricle. There here run across in it from one side of the brain to 

 the other several slender tracts of fibers. One of these, Meynerfs commis- 

 sure, is better defined than the others. Its origin and destination are not 

 sufficiently well known. It was retained in the dog after complete destruc- 

 tion of the cortex. Gudchn's commissure, lying anterior and ventral to it, 

 we will become better acquainted with later. In reptiles, Meynerfs com- 

 missure arises from the giant-celled nucleus of the central gray matter. 



The central gray matter on the mesial surface of the thalamus unites 

 with that of the opposite side for a distance of about three-fourths of a 

 centimeter to form the commissura mollis, or media. 



In man few medullated fibers run in it. Whether a commissure which 

 is present in lower vertebrates in an analogous location, and which is much 

 richer in fibers, is identical with the commissura media still remains to be 

 determined (see Fig. 83). 



Nissl has shown for the rabbit that each of the thalamic nuclei is again divided 

 into from three to four subnuclei, which are easily distinguishable from one another 

 by the behavior of their cells toward stains. Moreover, he has described in this 

 animal: a nucleus of the "latticed" layer and, anterior to the ganglion habenulse, 

 the nucleus of the median line. To these, then, there might still be added the small 

 nucleus magno-eellularis, found in the most anterior planes of the thalamus. Monakoio 

 has shown that the corpus geniculatum laterale is divided into five nuclei: a dorsal 



