690 



THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



fissure, and receiving in the middle of its inferior face the insertion of the 

 cerebral peduncles. 



This mass, seven to nine times more voluminous than the cerebellum, 

 fills the anterior compartment of the cranial cavity, and thus occupies the 

 greater portion of that space. 



It exhibits for study its external conformation, its internal conformation, 

 and its structure. 



Fig. 329. 



iff 



ANTERO-POSTERIOR AND VERTICAL SECTION OP THE ENCEPHALON TO ONE SIDE 

 OF THE MEDIAN LINE. 



1, 1, Isthmus of the encephalon ; 2, Medulla qblongata ; 3, Pons Varolii ; 4, Cerebral 

 peduncle ; 5, 6, Corpora bigemina ; 7, Optic thalamus ; 8, Pituitary gland ; 9, 

 Pituitary stem; 10, Optic nerve; 11, Cerebellum; 12, 12, Cerebral hemisphere; 

 13, Ventricle of the hemisphere ; 14, Corpus striatum 15, Cornu Ammonis ; 16, 

 Olfactory lobe ; 17, Ventricle of the olfactory lobe. 



EXTERNAL CONFORMATION OF THE CEREBRUM. 



Instead of examining the organ in mass, with regard to its external con- 

 formation, we will first consider the great interlobular (or longitudinal) fissure 

 which divides it lengthways ; and afterwards study its two lateral halves, 

 or cerebral hemispheres, which in reality constitute two symmetrical organs. 



1. The Longitudinal Fissure. 



This fissure exists throughout the vertical and antero-posterior circum- 

 ference of the cerebrum, but does not everywhere oner the same disposition. 

 On the superior aspect of the organ it is very deep, and when the two 

 hemispheres are separated to discover its extent, we see that ifc reaches to 

 the upper face of the great commissure the corpus callosum. Behind, it 

 curves between the posterior lobes of the hemispheres, but without corres- 

 ponding directly with the posterior thick rounded margin of the corpus 

 callosum, above which there is a feeble adhesion established between the 

 two halves of the cerebrum, forming a kind of bridge. But in front it passes 

 to the anterior margin of this commissure, and is prolonged in the interval 

 of the anterior lobes of the hemispheres to reach the inferior face of the 

 organ. 



Examined inferiorly, the interlobular fissure is well defined in front, 



