NERVOUS SYSTEM. 
127 
whose fibrils bestow sensibility on the face, except a few branches 
from the cervicals which may be traced to the lower part of it. 
The Spinal Marrow . 
Now then, gentlemen, we shall perhaps better understand our 
subject, if, for a little while, we leave the brain, and inquire into 
the construction of the nerves that supply the frame generally, 
and direct the voluntary motions of the muscles of the body, and 
bestow sensibility on every part; and these, as every tyro knows, 
can be traced on the trunk directly to the spinal marrow , and 
indirectly to it in the limbs, through the medium of the humeral 
and femoral plexuses. 
Commencement of the Spinal Marrow .—I have already said, 
that I cannot point out the particular spot where the medulla 
oblongata terminates, and the spinal marrow commences ; but 
I observe the medulla oblongata decreasing in width ; the cor¬ 
pora olivaria in the human being subsiding into the general level, 
and in the subjects with which we have to do, the comparatively 
flattened and extended portions at the side of the medulla ob¬ 
longata (answering to or comprising both the corpora olivaria and 
restiformia of the human being) also rapidly narrowing, and the 
diminution of width proceeding until I arrive at the spot, or 
nearly to it, where this prolongation of the brain quits the cra¬ 
nium and enters the spinal cavity :—this spot,—the mouth of the 
foramen magnum,—I consider to be the place where the medulla 
oblongata ends, and the medulla spinalis commences. 
Form and Appearance of the Spinal Marrow .—When exposed 
to view, the spinal marrow has the appearance of a flattened cy¬ 
linder, somewhat more convex interiorly than superiorly. It is 
small compared with the canal through which it runs. The 
brain completely fills the cranium :—it does so in sickness and 
in health, for however emaciated the frame generally may be¬ 
come, the absorbents spare the brain; or if from local disease a 
part of it wastes, the vacuity is occupied by some fluid. It was 
necessary that concussion should be avoided in so important an 
organ; but there could be no joints in the cranium for they 
would have interfered with its requisite strength $ and therefore 
concussion and injury were obviated by the cranium, being al¬ 
ways full. The spine is composed of a series of joints ;—the 
dorsal vertebrae admit of as much motion as is consistent with 
strength, and the cervical necessarily possess a greater degree of 
motion ; and consequently if the canal was perfectly occupied 
by the marrow, the chord would be seriously compressed and 
injured by the various, and rapid, and extensive flexures of the 
vertebral column. 
