THE BRAIN 653 



surface or l^ase presents the median brain stem, which is continuous with the spinal 

 cord without any natural line of demarcation; it divides in front into two l)ranches, 

 the cerebral peduncles, each of which disa]ip(^ars into the mass of the corresponding 

 cerebral hemis]:)here. The brain stem consists of three parts. The medulla 

 oblongata is the posterior jiart which extends forward as the direct continuation 

 of the spinal cord. The pons (Varolii) is a transversely elongated mass which 

 appears to turn up on either side into the cerebellum. The cerebral peduncles 

 extend forward from the pons and diverge to plunge into tlie ventral ]:)art of the 

 cerebral hemispheres. The area l)etween them is the interpeduncular space. It 

 is largely covered by the pituitary body or hypophysis, a yellowish-brown, discoid 

 structure, which is connected with the base of the cerebrum by a delicate tube 

 called the infundibulum/ On drawing the pituitary body gently aside, the 

 infundibulum is seen to be attached to a slight gray eminence, the tuber cinereum. 

 Behind this is the mammillary body, a well-marked round prominence. The 

 posterior part of the space is perforated by numerous openings for the passage of 

 small arteries, and hence is termed the locus perforatus posticus. Two large 

 bands of white matter, the optic tracts, cross the anterior ends of the cerebral 

 peduncles and unite at the optic chiasm or commissure, forming the anterior 

 boundary of the interpeduncular space. Above and in front of the chiasm the 

 hemispheres are separated by the median longitudinal fissure. In contact with 

 the anterior extremity of each hemisphere is the olfactory bulb, an oval enlargement 

 adapted to the ethmoidal fossa of the cranium. This a]i])ears as a gray swelling 

 on a wide flat band, the olfactory peduncle, which is continued behind by two diver- 

 gent bands, the olfactory tracts or striae. The internal tract disappears after a 

 very short course on to the inner surface of the hemisphere. The external tract 

 is larger and longer; it runs backward, inclines at first outward and then curves 

 inward and disappears on the concealed or tentorial surface of the hemispheres. 

 It is separated from the lateral cerebral gyri by a distinct groove (Sulcus rhinalis). 

 Along the inner side of the stria are two eminences. The anterior of these is the 

 trigonum olfactorium, a gray elevation situated in the angle of divergence of the 

 inner and outer stria^. Behind this is a depression, the fossa transversa," which is 

 continued across the external stria and sharply limits the second and much larger 

 eminence, the pyriform lobe. 



The superficial origins of most of the cranial nerves are visible on the base of 

 the brain. 



The olfactory nerve-fibers join the convex surface of the olfactory lobe and give 

 it a shaggy appearance in specimens which have been removed intact — a difficult 

 proceeding. 



The second or optic nerves converge to the optic chiasm. 



The third (jr oculomotor nerve arises from the inner part of the cereljral 

 peduncle. 



The fourth or trochlear nerve may be seen emerging between the pons and the 

 hemisphere, but its connection with the brain is not visible. 



The fifth or trigeminal nerve is connected with the lateral ]:)art of the pons. 



The sixth or abducent nerve arises just l^ehind the pons and lateral to the 

 pyramid of the medulla. 



The seventh or facial and the eighth or auditory nerves arise close together 

 just behind the pons proper on the extremity of the corpus trapezoideum. 



The ninth or glosso-pharyngeal, the tenth or vagus, and the eleventh or spinal 



' Unless care is used in removing the brain the infundibulum is likely to be torn and tlie 

 pituitary body left in the cranium. In this case there is a small opening which communicates 

 with the third ventricle. 



- The fossa corresponds in position to the locus perforatus anticus of the human brain, but 

 is not pierced by numerous openings for vessels in the horse. 



