Gross Axatomy 13 



of these forms the lateral wall of the anterior part of the 

 ventricle. 



Immediately behind the cut part of the peduncles, crossing 

 the restiform body, is a transverse ridge, the tuberculum 

 acusticum, which appears to become continuous ventro- 

 laterally with the eighth or auditory nerve. Anterior and 

 ventral to this, the facial nerve (MI) emerges from the side 

 of the medulla, and just anterior to it again is the root of the 

 trigeminal nerve (\'), in which large, sensory and small, 

 motor parts may be distinguished. 



In order that the midbrain may be observed the posterior 

 dorsal portions of the cerebral hemispheres must be raised 

 and pressed apart. This reveals two pairs of rounded emi- 

 nences, the corpora quadrigemina, which make up the dorsal 

 part of the midbrain. The ventral part is considerably 

 narrower antero-posteriorly than is the dorsal part, so that 

 the region is somewhat wedge-shaped. As seen from below, 

 it appears in a depression between the pons behind and the 

 cerebral hemispheres and mamillary body in front. A pair 

 of massive longitudinal fibre-tracts, the pedes pedunculi 

 cerebri, converge from the hemispheres and disappear under 

 the pons, covering the whole ventral aspect of the midbrain 

 except a median hollow between them, the interpeduncular 

 fossa. Through each peduncle emerges the oculomotor 

 nerve (III). 



In the forebrain — indeed in the brain as a whole — ^the 

 most conspicuous structures are the large cerebral hemis- 

 pheres, which in the rat and other small animals have smooth 

 surfaces, but which in the larger mammals are greatly con- 

 voluted. The hemispheres of the rat are much smaller pro- 

 portionately than those of man and the higher mammals. 

 They are separated by a deep median cleft, the longitudinal 

 cerebral fissure. 



At the front of each hemisphere lies the olfactory bulb, in 

 which end the numerous fine strands of the olfactorv nerve 



