THE CEREBRUM. 



783 



lobes or hemispheres of that apparatus — enlargements which are elongated in the 

 direction of the great diameter of the head and cranial cavity, lie beside each 

 other in the middle line, and are united at their central part by a transverse 

 commissure, and by the isthmus, the anterior extremity of which penetrates 

 their substance inferiorly (see Figs. 424, 429, 432 for a good idea of this 

 penetration). 



These two lobes together represent an ovoid mass, having its larger extremity 

 adjacent to the cerebellum ; it is depressed above and below ; deeply divided 

 above, in front, and behind by a median antero-posterior tissure, and receives in 

 the middle of its inferior face the insertion of the crura cerebri. 



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. 



Leuret has found that, on the average, it weighs in the Stallion, 15^ ounces ; 

 in the Mare, 14i ounces ; and in the castrated Horse, 14 ounces 12i drams. 



Fiff. 432. 



ANTERO-POSTERIOR AND VERTICAL SECTION OF THE BRAIN, TO ONE SIDE OF THE MEDIAN LINE. 



1, 1, Isthmus; 2, medulla oblongata; 3, pons Varolii; 4, crus cerebri; 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 bulb; 17, ventricle in the olfactory bulb. 



It offers for study its external conformation, its internal conformation, and its 

 structure. 



ExTEENAL Conformation of the Cerebrum. 



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

 tion, we will first consider the great longitudinal fissure which divides it length- 

 ways ; and afterwards study its two lateral halves, or cerebral hemispheres, which 

 in reality constitute two symmetrical organs. 



1. The Longitudinal Fissure (Figs. 423, 424), 



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

 ference of the cerebrum, but does not everywhere offer the same arrangement. 



