196 ANATOMY OF THE CENTRAL NERVOUS SYSTEM. 



ward behind the lobulus parietalis inferior. One or two small sulci, placed 

 somewhat horizontally, separate the small gyri from one another. 



When all of these fissures and convolutions have been found, cut the 

 brain in two along the line of the great longitudinal fissure, and study the 

 mesial side of the hemisphere. 



The most important parts of the mesial wall of the hemisphere are 

 already familiar to you from the study of their embryology in the second 

 chapter. It was there learned that the edge of the hemisphere, thickened 

 to form the fornix, follows in curved line the hemisphere, which grows out 

 posteriorly and inferiorly; that anteriorly, where the corpus callosum passes 

 across from one hemisphere to the other, the portion of the inner wall which 

 lies between the fornix and the corpus callosum remains as the septum 

 pellucidiTm. 



From its embryology, the section previously made through the brain of 



Fig. 132. — Inner aspect of the embryonic hemisphere shown in Fig. 23. It 

 shows the inner, inferior edge of the hemisphere, which becomes thiclcened to 

 form the white, meduUated line of the fornix. This, however, becomes meduUated 

 after birth only. Zifisclirnliini, Interbrain. Torderliirn, forebrain. iitcIJc, etc., 

 Place where forebrain and interbrain meet. 



the adult is easily understood. In the preparation from which the accom- 

 panying illustration was made (Fig. 133), as well as on the embryonic brain 

 now again demonstrated (Fig. 132), all the parts that lie behind the middle 

 of the thalamus have been cut away, because they conceal the imder surface 

 of the temporal lobe and prevent us from following the fornix. 



The interbrain — that is, its lateral wall, the thalamus opticus — is there- 

 fore now observed in the center on the longitudinal section. The margin of 

 the hemisphere, thickened to form a white, medullary band, the fornix, 

 passes in a curved line along the boundary between the interbrain and the 

 cerebrum. It first appears near the base of the brain in the gray matter 

 behind the lamina terminalis: ascends dorsally as the columna fornicis; 



