_a_, Spinal cord; _b_, dorsal median groove; _c_, _c′_, funiculus
gracilis; _c″_, clava; _d_, _d′_, funiculus cuneatus; _d″_,
tuberculum cuneatum; _e_, funiculus lateralis; _e′_, tuberculum
Rolandi; _f_, medulla oblongata; _I_, _II_, _III_, floor of fourth
ventricle (fossa rhomboidea); _g_, limiting groove; _h_, median
fissure; _i_, eminentia medialis; _k_, ala cinerea; _l_, _l′_, pons;
_m_, posterior peduncle, and _n_, middle peduncle of cerebellum
(cut); _o_, anterior peduncle of cerebellum; _p_, _p′_, corpora
quadrigemina, anterior and posterior; _q_, _q′_, peduncles (brachia)
of corpora quadrigemina; _r_, thalamus; _s_, massa intermedia; _t_,
stria medullaris; _u_, anterior tubercle of thalamus; _v_, pulvinar;
_w_, corpus geniculatum laterale; _x_, corpus geniculatum mediale.
(After Ellenberger, in Leisering’s Atlas.)
]
The =brain= weighs about 60 to 70 g. in dogs of medium size, but there
is, of course, a wide range of weight in the different breeds. Thus in
small terriers the weight is 30 g. or even less, while in very large
dogs it may exceed 150 g. It corresponds much more closely with the
cranium in size and general form than in the animals previously
described; this is specially true of the small breeds, in which the bony
crests and frontal sinuses are little developed.
The =medulla= is broad and thick. Its ventral surface is strongly convex
from side to side. The pyramids are large and prominent and are limited
by distinct median and lateral grooves. The olivary eminence is a well
defined oval elevation situated between the pyramid and the superficial
origin of the hypoglossal nerve. The external arcuate fibers form a wide
band which crosses the lateral surface obliquely upward and forward, and
obscures the groove which limits the restiform body externally. The
cuneate tubercle is distinct. The rhomboid fossa is deep and narrow.
[Illustration:
FIG. 541.—PART OF CROSS-SECTION OF BRAIN OF DOG, PASSING THROUGH
GANGLION HABENULÆ.
_Al_, Ansa lenticularis of thalamic radiation; _C a d_, dorsal end of
hippocampus; _C a v_, ventral end of same; _C c_, corpus callosum;
_C g m_, corpus geniculatum mediale; _C m_, corpus mammillare; _F i
d_, fimbria; _F i v_, ventral part of fimbria; _G h_, ganglion
habenulæ; _M B_, fasciculus retroflexus; _P p_, pes pedunculi; _T
h_, thalamus; _T o_, tractus opticus; _T t_, habenula; _V A_,
fasciculus thalamo-mammillaris; _V F_, fornix ventralis; _II_,
lateral ventricle; _III_, third ventricle. (After Probst.)
]
The =pons= is relatively small. The =corpus trapezoideum= is very wide
and is divided by the pyramids into two lateral parts. The =cerebellum=
is very broad, but is low and also compressed from before backward.
Rather more than half of it is overlapped by the cerebral hemispheres.
The anterior surface is accurately adapted to the concave tentorial
surfaces of the hemispheres and to the posterior corpora quadrigemina
and their commissure. The posterior surface is almost vertical and is
convex centrally and flattened laterally. The vermis is prominent and in
general well defined, although it is connected in its middle part with
the hemispheres. The latter are three-sided and consist of four lobules.
The anterior peduncles are very short.
The =posterior corpora quadrigemina= are large, very wide apart, and
prominent, and are connected by a curved commissure. The =internal
geniculate body= is large. In the deep interpeduncular fossa are two
small bands which indicate the course of the fasciculi retroflexi,
tracts which connect the habenular and interpeduncular ganglia. The
=mammillary body= is double. The =tuber cinereum= is relatively large.
The =pituitary body= is circular and rather small.
The =cerebral hemispheres= are very broad behind and diminish in width
anteriorly; there is a sudden narrowing at the frontal poles, which are
flattened laterally. The tentorial surfaces are concave and form a deep
cavity which receives the anterior part of the cerebellum. The
arrangement of the fissures and gyri is simpler than in the animals
previously examined. The chief fissures are as follows:
1. The =lateral fissure= (of Sylvius) extends upward and backward from
the sulcus rhinalis a little behind the middle of the lateral surface of
the hemisphere.
2. The =ectosylvian fissure= has approximately the shape of an inverted
u, and curves over the lateral fissure. It is regarded as consisting of
three parts—anterior, middle, and posterior (F. ectosylvia anterior,
media, posterior).
3. The =suprasylvian fissure= lies above and approximately concentric
with the preceding. It also consists of three parts—anterior, middle,
and posterior.
4. The =ectomarginal= or =collateral fissure= runs forward on the dorsal
aspect of the hemisphere almost parallel with the dorso-medial border. A
little in front of the middle of the surface it gives off a short
=ansate fissure= (F. ansata) which runs obliquely inward and forward,
and is continued by the coronal fissure.
[Illustration: