242 ANATOMY OF THE CENTRAL NERYOUS SYSTEM. 



most mesial radiation of the fibers of the forceps. Lately, however, the investiga- 

 tions of the authors cited above, in connection with the fasciculus fronto-occipitalis, 

 make it probable that the tapetum does not radiate into the corpus callosum; rather 

 that it is the most posterior radiation of that long association-tract. The radiation 

 of the fasciculus fronto-occipitalis lies directly beneath the epithelium of the ven- 

 tricle, next to the cavity, and only outside of it lie the posterior radiations of the 

 corpus callosum. 



For all that, it appears to me that numerous fibers of the corpus callosum are 

 mixed with these. At least, the relatively small medullated bundles that form the 

 fasciculus fronto-occipitalis in the dog do not cover all the numerous fibers of the 

 tapetum. The fact that the tapetum has been observed to be present in cases where 

 the corpus callosum was A^'anting, and the fact that it does not degenerate after 

 section of the corpus callosum, appear to be the most important reasons for per- 

 mitting it to be separated from the corpus callosum. 



In the osmatic mammals, where the cornu Ammonis extends upward as far as 

 the under surface of the corpus callosum, it is recognized more clearly than in man 

 that the posterior end of the corpus callosum curves forward again. In this man- 

 ner it forms a distinct layer of fibers dorsal to the psalterium, which, in sections, is 

 to be distinguished from the fibers arising from the cornu Ammonis by the smaller 

 diameter of its fibers only. This portion of the corpus callosum, as well as the dorsal 

 portion, is penetrated by the bundles of the fornix longus (Fig. 144). 



The accompanying illustration (Fig. 158) represents the fibers of the 

 corpus callosum when exposed from the inner surface of the brain. With 

 its help, an accurate idea may be easily formed of the radiation of the corpus 

 callosum. 



The commissura anterior has already been described in connection with 

 the description of the olfactory apparatus. In man it passes as a large 

 bundle of fibers, close to the fioor of the ventricle, in front of the pillars of 

 the fornix. It cannot be followed in a transverse section in the manner 

 intimated in the semidiagrammatic figure here. On each side its fiber-mass, 

 while it passes through the corpus striatum, curves much more, downward 

 and backward, in a semicircle and is lost in the most posterior portion of the 

 medulla of the lobus temporalis. In Fig. 127 this curved portion is cut on 

 both the right and left sides below and external to the nucleus lentiformis. 



The greatest part of the anterior commissure in man contains only fibers 

 that can be traced backA\-ard until in the region lateral to the cornu Ammonis 

 (compare Fig. 154). A small bundle only of the commissure of the olfactory 

 lobe is shown, — it is seen passing downward in Fig. 157. 



Numerous fibers arise from all parts of the cortex of the forebrain, 

 which connect the forebrain with the deeper-lying parts of the central nerv- 

 ous system. A great many pass into the interbrain; others may be traced as 

 far as the gray masses of the midbrain and as far as into the nerve-nuclei 

 of the pons, in which they appear soon to terminate. Some of them pass 

 down through the internal capsule, the crus cerebri, the pons and the 



