458 THE BRAIN 



Oculo - motor Nerve. This may be seen within the 

 interpeduncular fossa. It issues by several fila from the 

 sulcus oculomotorius on the medial face of the cerebral 

 peduncle. 



Optic Nerve. This is a large round nerve which passes 

 antero-laterally from the optic chiasma. 



Olfactory Nerves. These arise from the olfactory bulb 

 and enter the nasal cavity through the foramina in the 

 cribriform plate of the ethmoid bone. 



General Connections of the Several Parts of the Brain. 

 Before proceeding to the more particular study of the different 

 parts of the brain the student should acquire a general 

 conception of the manner in which these are connected with 

 each other. In the posterior cranial fossa, below the ten- 

 torium, are placed the medulla oblongata, the pons, and 

 the cerebellum, which constitute collectively the Rhomben- 

 cephalon or hind-brain. They surround a cavity which is 

 called the fourth ventricle of the brain, and all stand in 

 intimate connection with one another. The upper end of 

 the medulla oblongata terminates chiefly in the pons, but two 

 large strands on its posterior or dorsal aspect, termed the 

 restiform bodies, pass posteriorly into the cerebellum (Fig. 

 185, d). The transverse fibres of the pons are gathered 

 together on either side in the form of a large rope-like strand, 

 the brachium pontis. This disappears into the corresponding 

 hemisphere of the cerebellum (Fig. 185, c\ 



The great mass of the brain is termed the cerebrum. 

 This occupies the anterior and middle cranial fossae, and 

 extends posteriorly into the occipital region above the 

 tentorium cerebelli. The greater part of the cerebrum is 

 formed by the cerebral hemispheres, which are separated 

 from each other, in the median plane, by the longitudinal 

 fissure. At the bottom of this fissure may be seen the corpus 

 callosuin, a broad commissural band which connects the two 

 cerebral hemispheres with each other. Each hemisphere is 

 hollow the cavity in its interior being termed the lateral 

 ventricle of the brain. Between and below the cerebral 

 hemispheres, and almost completely concealed by them, is 

 the thalamencephalon. The principal parts forming this portion 

 of the brain are the two thalami, between which is the third 

 ventricle of the brain a deep, narrow cavity occupying the 

 median plane. The third ventricle communicates with the 



