THE GENERAL TOPOGRAPHY OF THE BRAIN 117 



another. The medulla oblongata is directly continuous with the pons, and on 

 either side a large bundle of fibers from the dorsal aspect of the former runs into 

 the cerebellum. These two strands, which are known as the restiform bodies 

 or inferior cerebellar peduncles, constitute the chief avenues of communication 

 between the spinal cord and medulla oblongata on the one hand and the cere- 

 bellum on the other. The ventral prominence of the pons is produced in large 

 part by transverse bundles of fibers, which when traced lateralward are seen to 

 form a large strand, the brachium pontis or middle cerebellar peduncle, that 

 enters the corresponding cerebellar hemisphere (Figs. 83, 86). The brachium 

 conjunctivum or superior cerebellar peduncle can be traced rostrally from the 

 cerebellum to the mesencephalon. The three peduncles are paired structures, 

 symmetrically placed on the two sides of the brain (Figs. 87, 88). 



The Cerebrum. The mesencephalon surrounds the cerebral aqueduct and 

 consists of the ventrally placed cerebral peduncles, and a dorsal plate with four 

 rounded elevations, the lamina and corpora quadrigemina (superior and inferior 

 colliculi). The cerebral hemispheres form the most prominent part of the 

 cerebrum and are separated from each other by the longitudinal fissure (Fig. 

 82), at the bottom of which is a broad commissural band, the corpus callosum, 

 which joins the two hemispheres together (Fig. 85). Under cover of the cere- 

 bral hemispheres and concealed by them, except on the ventral aspect of the 

 brain, is the diencephalon. This includes most of the parts which help to form 

 the walls of the third ventricle. These are from above downward, the epithal- 

 amus, including the habenular trigone and pineal body near the roof of the 

 ventricle; the thalamus, which forms most of the lateral wall of the ventricle, 

 and is united with its fellow across the cavity by a short bar of gray substance, 

 the massa intermedia; and the hypothalamus, including the mammillary bodies, 

 infundibulum, and part of the hypophysis (Figs. 84, 85). 



The Brain Ventricles. The central canal of the spinal cord is prolonged 

 through the caudal portion of the medulla oblongata and finally opens out into 

 the broad rhomboidal fourth ventricle of the rhombencephalon. At its pointed 

 rostral extremity this ventricle is continuous with the cerebral aqueduct, the 

 elongated slender cavity of the mesencephalon. This, in turn, opens into the 

 third ventricle, which is a narrow vertical cleft between the two laterally sym- 

 metric halves of the diencephalon. It is bridged by the massa intermedia. 

 Near the dorsal part of the rostral border of the ventricle is a small opening in 

 each lateral wall, the inter-ventricular foramen or foramen of Monro. This 

 leads into the lateral ventricle, the cavity of the cerebral hemisphere. 



