The =brain= or =encephalon= is the part of the central nervous system
that is situated in the cranial cavity. It is the enlarged and highly
modified cephalic part of the primitive neural tube. It conforms rather
closely in size and shape to the cavity in which it lies.
Its average weight without the dura mater is about 23 ounces (ca. 650
gm.), and forms about ⅐ of 1 per cent. of the body-weight.
It is desirable to examine the general external configuration of the
brain before studying its various parts in detail.[187]
When divested of its membranes and vessels (Figs. 499, 500), its ventral
surface or base presents the median =brain stem=, 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= (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 the
ventral part of the cerebral hemispheres. The area between 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=.[188] 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 appears as a gray swelling on a wide flat band, the
=olfactory peduncle=, which is continued behind by two divergent bands,
the =olfactory tracts= or =striæ=. 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 striæ. Behind this is a depression,
the =fossa transversa=,[189] 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= or =oculomotor nerve= arises from the inner part of the
cerebral 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 part of
the pons.
The =sixth= or =abducent nerve= arises just behind 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 accessory nerves= are connected by a linear series
of roots with the lateral aspect of the ventral surface of the medulla.
The spinal part of the accessory nerve comes forward along the edge of
the medulla to join its medullary root.
The =twelfth= or =hypoglossal nerve= arises from the posterior part of
the medulla along the lateral edge of the pyramid.
[Illustration: