THE SYMPATHETIC SYSTEM. 443 
less in the female. Taking into consideration that the body of the horse 
weighs at least eight times as much as a man’s, it follows that the brain 
of the latter is relatively sixteen times as large as that of the horse. 
The cerebellum occupies the postero-superior part as the head is usually 
carried, and is much smaller than the cerebrum, being only one-sixth of 
its volume. Examining it from above it presents three lobes ; a middle 
and two lateral lobes. The former is prominent, and subdivided into 
jobuli by several grooves, constituting the anterior and posterior vermiform 
processes. The lateral lobes are flattened and oval, to correspond with 
the inferior surfaces of the tentorium cerebelli. The cerebellum is made 
up of alternate layers of grey and white matter, the former being dis- 
tributed throughout the interior in such a way that when sliced it presents 
an arborescent appearance. On parting the hemispheres of the cerebrum, 
the convolutions on the surface of which are composed of grey matter, a 
white band slightly striped from side to side makes its appearance. This 
is the corpus callosum, which is the great commissure, and consists 
entirely of white fibrous matter, uniting the two halves. Beneath this, 
on each side, are the lateral ventricles, and within each are the optic 
thalamus, and corpus striatum, with the choroid plexus lying between. 
Turning the brain with its inferior surface in view it presents anteriorly 
the continuation of the longitudinal fissure. On each side of this are 
the olfactory nerves, which look like prolongations of the hemispheres. 
Close behind these two are the optic or second pair of nerves, connected 
together by their commissure. Then two small white bodies, the corpora 
albicantia, and behind these again the third pair of nerves, supplying the 
muscles of the eye. Still further back is a square eminence, the pons 
varolii, from the sides of which the fourth and fifth pairs of nerves arise, 
while its posterior border gives origin to the sixth pair in the middle and 
the seventh externally to these. The eighth and ninth nerves have 
already been alluded to, as arising from the medulla oblongata, 
THE SYMPATHETIC SYSTEM. 
THIS DIVISION OF THE NERVES consists of a series of ganglia, lying 
on each side the spine, from the head to the coccyx, communicating with 
the cranial and spinal nerves, and distributing branches to all the internal 
organs of digestion, circulation, depuration, and generation. The branches 
of distribution accompany the arteries, forming a plexus, or series of 
meshes, around each of them. In the head there are four small ganglia, 
in the neck three, and posteriorly a small ganglion lies opposite each 
vertebra. The posterior cervical ganglion communicates with the spinal 
nerves of that region by a branch which accompanies the vertebral artery, 
and sends forward filaments to form the bronchial and cardiac plexus, the 
former being largely supplied also with branches from the pneumogastric 
nerve. From the dorsal ganglia a large nerve is formed, the greater 
splanenic nerve, and also the lesser splancnic, which enter the abdomen 
close beneath the crus of the diaphragm, where they give off a number of 
branches which, together with filaments of the pneumogastric nerve, unite 
on both sides to form the semilunar ganglion, or collection of ganglia 
arranged somewhat in that shape. They lie close to the posterior aorta, 
and surround the root of the cceliac artery, supplying branches to form 
the phrenic and the splenic plexus, the gastric plexus, the hepatic plexus, 
the anterior and posterior mesenteric plexus, the renal plexus, and the 
spermatic plexus, all surrounding the corresponding arteries and supplying 
the important organs whose names they bear. 
