THE BRAIN 769 



23 ounces (ca. 650 gm.), and forms about | of 1 per cent, of the body-weight in a 

 subject of medium size. 



It is desirable to examine the general external configuration of the brain before 

 studying its various parts in detail.i 



When divested of its membranes and vessels (Fig. 629), its ventral surface or 

 base presents the median brain stem (Caudex encephali), which is continuous with 

 the spinal cord without any natural line of demarcation; it divides in front into 

 two branches, the cerebral peduncles, each of which disappears into the mass of 

 the corresponding cerebral hemisphere. The brain stem consists of three parts. 

 The medulla oblongata is the posterior part which extends forward as the direct 

 continuation of the spinal cord. The pons is a transversely elongated mass which 

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

 (Pedunculi cerebri) extend forward from the pons and diverge to plunge into the 

 ventral part of the cerebral hemispheres. The area between them is the inter- 

 pedimcular fossa. It is largely covered by the hypophysis cerebri or pituitary 

 body, a yellowish-brown, discoid structure, which is connected with the base of 

 the cerebrum by a delicate tube called the infxmdibulum.^ On drawing the pitu- 

 itary body gently aside, the infundibulura is seen to be attached to a slight gray 

 eminence, the tuber cinereum. Behind this is the mammillary body (Corpus 

 mamillare), 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 substantia perforata posterior. A large band of white matter, the 

 optic tract (Tractus opticus), crosses the anterior end of the cerebral peduncle ob- 

 liquely, and unites with the opposite tract to constitute the optic chiasm or commis- 

 sure (Chiasma opticum), and form the anterior boundary of the interpeduncular 

 fossa. 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 (Bulbus olfactorius), an oval enlargement which 

 occupies the ethmoidal fossa of the cranium. This appears as a gray swelling 

 on a wide fiat band, the olfactory tract or peduncle (Tractus olfactorius), which 

 is continued behind by two divergent bands, the olfactory striae. The medial 

 stria (Stria medialis) disappears after a very short course on to the medial surface 

 of the hemisphere. The lateral stria (Stria lateralis) is larger and longer; it runs 

 backward, inclines at first outward and then curves medially 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 medial 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 medial and lateral striae. 

 Behind this is a depression, the fossa lateralis, which is continued across the lateral 

 stria and sharply limits the second and much larger eminence, the piriform 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 bulb 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 or oculomotor nerve arises from the medial part of the cerebral 

 peduncle. 



1 The description given here is intended to present the chief facts in regard to the brain as 

 they may be studied in the dissecting-room. The vessels and membranes, which must be exam- 

 ined first, have been described. 



2 Unless care is used in removing the bram, the infundibulum is likely to be torn and the 

 hypophysis left in the cranium. In this case there is a small opening which communicates with 

 the third ventricle. 



49 



