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. 
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 peduncles 
(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- 
peduncular 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 infundibulum.” On drawing the pitu- 
itary body gently aside, the infundibulum 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 flat 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 striz. 
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. 
‘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- 
: i 5 ’ 
ined first, have been described. 
* Unless care is used in removing the brain, 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. 
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