ON THE CEREBRAL NERVES. 
581 
the various parts of the frame. There are difficulties on both 
sides. The nerves of pure sensation seem to rise from, or to be 
prolongations of, the substance of the brain—the nerves of com¬ 
mon sensation appear to spring bodily from the matter of the 
brain—the nerves of motion would seem more to be spread upon 
the surface of the brain, and to be easily torn off. Whatever be 
the fact, I must, to render myself intelligible, adopt the popular 
language, and consider the nerves as emanations from the brain. 
The Nerves are Symmetrical. —We cannot regard the base of 
the brain for a moment, without observing how regularly they 
arise by pairs from each side of the mesian line, which, com¬ 
mencing in the brain, pervades the whole frame. There are two 
complete systems of nerves. It is a kind and wise provision in 
order that the senses necessary to the comfort, if not the exist¬ 
ence of the animal, should not be destroyed by slight and partial 
injury. 
Comparative Size. —These nerves are, without exception, more 
developed in the quadruped than in the human being. Compare 
the nerve of smelling, of sight, or of hearing, in the two. This 
acuteness of sense was necessary partly for the existence of the 
animal, but more to qualify him to be our useful servant. We 
have that, the most valuable of all the senses, which he seems 
comparatively to want, that of touch, and, what is yet more 
important, we have increased power of intellect. 
The Neurilema. —Every nerve, as it proceeds from the brain 
or spinal chord, is covered by a theca or sheath. It is difficult to 
describe the nature of this neurilema. It most resembles, or, 
proceeding from the spinal chord, is continuous with, the spinal 
pia mater. It is denser than the parietes of the vein, or even the 
artery. In the operation of neurotomy there is sometimes no 
little difficulty in dividing the theca. The sheath is necessary to 
give support to the pulpy matter of the nerve, and that it may lie 
in contact with and be compressed by the neighbouring parts 
without giving pain. When the nerve has reached its point of 
destination, or is divided into its minutest ramifications, the 
theca gradually becomes so thin, that the medullary substance is 
sensible of the presence and agency of these objects to which it 
was destined to respond—until then it is securely defended. 
Substance of the Nerves. —The substance of the nerves resem¬ 
bles much, or seems to be identical with, that of the brain. If, in 
our anatomical preparations, there seems to be a difference; if 
we easily preserve the nerves while the medullary matter rapidly 
putrefies, we should perhaps recollect, that it is the theca and not 
the substance of the nerve which is thus incorruptible. The 
nerves being deprived of their theca, or the soft nerves being 
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