CHAPTEE XVIII. 



The Base oe the Bhain. The Optic Nebye and its Oeigin. 

 The Cohpoka Quadkigemina. 



"We have hitherto taken no occasion to examine the base of the brain 

 more thoroughly. Now, when the origin of several of the structures found 

 there is known to us, it may be time gently to free a brain, with the base 

 turned upward, from the pia and blood-vessels, and to study the preparation 

 so made. 



The illustration that follows may serve as a guide. First of all, the 

 crura cerebri are seen to emerge from out the mass of the cerebrum. Just 

 in front of them, in the space here concealed for the greater part by the 

 optic nerve, lies the substantia innominata, which contains the ansa lenti- 

 formis and the inferior thalamic pedicle (see Pig. 174 also). Frontal sec- 

 tions, previously demonstrated, have taught you that the white mass here 

 visible, the pes, is the direct continuation of the fibers of the internal cap- 

 sule. After a short course the crus cerebri is covered by thick masses of 

 fibers, which appear to pass transversely across it from one-half of the cere- 

 bellum to the other. These fibers are designated as the pontal fibers, or filra 

 pontis. On the other side of the pons a part of the fibers contained in the 

 pes pedunculi can again be seen as the pyramids, another part has termi- 

 nated in the ganglia that are scattered in between the fibers of the pons. 



The gray matter between the crura cerebri is called the substantia per- 

 forata posterior. It borders internally on the regio subthalamica. In front 

 of it lie the corpora mamillaria, those two roundish ganglia that we have 

 previously met with in transverse section: the same ganglia to which the 

 bundle of Vicq d'Azyr passes from the thalamus, the ganglia in which 

 the fornix ends. 



In front of the corpora mamillaria the floor of the middle ventricle, 

 which is here designated as the tuber cinereum, bends, or bulges, downward 

 and forward — so that a funnel arises, the lumen of which is nothing but the 

 continuation of the ventricle. At the lower, pointed end of this funnel, the 

 infundihulum, the hypophysis is attached (see Fig. 162). 



The hypophysis — an appendix 'to the base of the brain, about the size of a 

 cherry — consists first of all of the continuation of the ventricular floor, the lobus 

 infundibuli, or lobus posterior, which is not positively known to be of a nervoua 

 nature. In front of this lies the anterior lobe, a tuft of epithelial tubules, which has 

 grown firmly to the lobus infundibuli, and which, as you know, arises from the 



(280) 



