BRAIN 421 



more independent subdivision of the hemisphere is the olfactory lobe, 

 which terminates anteriorly in the olfactory bulb an expansion which 

 receives the olfactory nerves. The entire olfactory portion of the brain 

 is called the rhinencephalon. 



Connecting the hemispheres with one another, there is a great transverse 

 commissure known as the corpus callosum (Fig. 434, c.c.). Below this is 

 the arched body of the fornix (f), representing a median fusion of two longi- 

 tudinal bundles of commissural fibers, only small parts of which are in- 

 cluded in a median section. Between the corpus callosum and the for- 

 nix, there is a thin septum pellucidum which consists of two vertical plates 

 with a closed cleft-like cavity between them. 



It is probable that the corpus callosum and body of the fornix develop in a thicken- 

 ing of the front wall of the telencephalon, where it crosses the median line. The cavity 

 of the septum pellucidum is, accordingly, a secondary cleft in the thickened wall. 

 A fusion between the adjacent medial walls of the hemispheres, to provide a path for 

 the fibers of the corpus callosum and to account for the cavity in the septum, has been 

 described, but not confirmed. 



In addition to the hemispheres with their commissures and olfactory 

 lobes, and the optic vesicles which are not counted as a part of the brain, 

 the telencephalon produces the pars optica hypothalami. This "optic 

 portion of the region below the thalamus" includes the optic recess, and 

 in the mid-ventral line it forms a funnel-shaped depression, the infun- 

 dibulum, terminating below in the posterior lobe of the hypophysis. (The 

 anterior lobe of the hypophysis is derived from the pharynx.) The 

 median cavity of the telencephalon is a laterally compressed space which 

 forms the front part of the third ventricle. The lateral ventricles, which 

 open from it, are counted as the first two. 



Diencephalon. In the mid-dorsal line the diencephalon produces a 

 cone-like body, the corpus pineale. Laterally, in its thick walls, there 

 is a mass of gray substance called the thalamus (bed). External to the 

 thalamus are the great bundles of fibers passing from the hemispheres 

 to the spinal cord. The sensory fibers ascending from the cord terminate 

 in the thalami, where there is a relay of nerve cells to convey the impulses 

 to the hemispheres. The thalami have other connections of equal impor- 

 tance. They come in contact with one another across the cleft-like cavity 

 of the diencephalon (which is a part of the third ventricle) and may fuse, 

 forming the massa intermedia. The ventricle surrounds this mass. Be- 

 neath the thalamus the diencephalon forms the pars mammillaria hypo- 

 thalami, which is represented on the under surface of the brain by the pair 

 of rounded mammillary bodies, one on either side of the median line 

 (Fig. 435, B). 



Mesencephalon. The mid-brain remains undivided, and its walls 

 become very thick. Dorsally it forms four rounded elevations, the 



