HEMISPHERES OF THE BRAIN. 



657 



cinereum, and connected with each other across the mesial plane. They are 

 mainly formed by the anterior crura of the fornix, which, after descending to the 

 base of the brain, are twisted upon themselves to form loops, and constitute the 

 white covering of the corpora albicantia. A second fasciculus, the bundle of 

 Vieq d 1 Azyr, converges from the optic thalamus, and enters the anterior part of 

 each body on its dorso-mesial surface. They are composed externally of white 

 substance, and internally of gray matter ; the nerve-cells of the gray matter are 

 arranged in two sets, inner .and outer, the cells of the former set being the smaller. 

 They are also connected to the tegmentum by a small bundle of fibres, the peduncle 

 of the mammillary body. At an early period of foetal life they are blended 

 together into one large mass, but become separated about the seventh month. In 

 most vertebrates there is only one median corpus albicans. 



The posterior perforated space (pons Tarini) corresponds to a whitish-gray 

 fossa placed between the corpora albicantia in front, the pons Varolii behind, and 

 the crus cerebri on either side. It forms the posterior part of the floor of the 

 third ventricle, and is perforated by numerous small orifices for the passage of 

 the postero-median ganglionic branches of the posterior cerebral and posterior 

 communicating arteries. 



Gray matter 

 White matter. 



Ventricle of 

 corpus callosum. 



Corpus callosum. 



Hippocampus 



Pons Varolii. 

 Cerebellum. 



Auditory nerve. 



Pneumogastric 

 nerve. 



FIG. 351. Transverse vertical section of the brain, through the fore part of the foramen magnum, looked 

 at from the front. (After Hirschfleld and Leveille.) 



The pons Varolii is situated immediately behind the two crura of the cerebrum. 

 It consists of a broad band of white fibres, which pass transversely from one cere- 

 bellar hemisphere to the other ; the band becoming narrower as it enters the cere- 

 bellum. In the middle line on its under surface a narrow groove runs from before 

 backward and accommodates the basilar artery. 



The medulla oblongata emerges from the posterior border of the pons Varolii ; 

 it is pyramidal in form, and is continuous below with the cervical portion of the 

 spinal cord. It is marked on its ventral surface by a median fissure, continuous 

 below with the anterior median fissure of the cord, and on either side by secondary 

 fissures and columns, which will be described in the sequel. 



The Frontal Lobe. The under surface of the frontal lobe, sometimes named 

 the orbital lobe, is seen on the anterior part of the base of the brain on either side 

 of the median line. It has already been described (page 649). 

 42 



