NERVOUS SYSTEM OF VERTEBRATA. 
363 
is so much higher, and its importance so much greater, that 
special care is taken to guard it from injury.—The term brain 
is commonly applied to the whole mass of nervous matter 
contained within the cavity of the skull; but this consists of 
several distinct parts, which have obviously different charac¬ 
ters. The principal mass in Man and the higher Yertebrata 
is that which is termed the Cerebrum (tig. 195, a) ; this occu¬ 
pies all the front and upper part of the cavity of the skull, and 
is divided into two halves or hemispheres by a membranous 
partition which passes from back to front along the middle 
line. Beneath this, at the back part of the skull, is another 
mass, b , much smaller, but still of considerable size, termed 
the Cerebellum; and this also is divided into two hemi¬ 
spheres. At the base or under side of the cerebrum, and 
completely covered-in by it, are two pairs of ganglia (1 and g , 
fig. 196), which belong to the nerves of smell and sight. We 
shall presently find that these are, relatively speaking, much 
larger in the lower Yertebrata than in the higher. 
450. The several masses of nervous matter contained in the 
skull, are connected with each other and with the spinal cord 
by bands of nerve-fibres and tracts of vesicular substance, 
which serve to bring the brain into connexion with the nerve- 
trunks issuing from the spinal cord. But the Spinal Cord 
has also distinct properties of its own, analogous to those 
which have been shown to exist in the chain of ganglia in 
Insects. The upper part of it, which passes-up into the 
cavity of the skull, is termed the Medulla Oblongata (/', fig. 
197). This is connected with the nerves of respiration, masti¬ 
cation, and deglutition; and may be regarded as combining 
together the respiratory and the stomato-gastric systems of 
Invertebrata. The remainder of the spinal cord, which de¬ 
scends through the vertebral column, sends its nerves to the 
limbs and trunk; and may be regarded as analogous to the 
chain of ganglia by which the corresponding parts are sup¬ 
plied in Insects. 
451. The nerves which issue from the Spinal Cord, all 
possess Wo sets of roots ; one from the anterior portion of 
the cord, the other from its posterior portion (fig. 191). The 
fibres which come-off by these two sets of roots, soon unite 
into the trunk of the nerve, which thus possesses the proper¬ 
ties common to both. It was the great discovery of Sir 
