NERVOUS SYSTEM. 
67 
chord. I trace it from the slit of the calamus scriptorius, and 
therefore from the centre of the surface. Its pulpiness and trans¬ 
parency render it now and then a little difficult to follow, and we 
often mutilate or lose it in the dissection ; but you here plainly per¬ 
ceive it passing down the side of the medulla oblongata, meeting 
the seventh nerve (the portio dura of the seventh), and then, in 
union with it, taking an outward direction,and entering the foramen 
auditorium internum. Its course there is short: it soon abuts 
upon a cribriform or perforated plate of bone closing the passage, 
through which it passes, and then, dividing into various portions, 
spreads itself over the internal chambers of the ear, while there 
is not throughout the whole of its course one anastomosis . Its 
root cannot, like the first and second nerves, be traced to distant 
parts, for it at first appears on that track of medullary matter to 
which the others are ultimately referred. It does not seem so 
much a prolongation of a part of the brain, and yet it springs 
as it were, bodily , and at once , from the floor of the ventricle. 
Human anatomists have spoken of fasciculi of fibres proceeding 
from above and below to form the root: something like striae I 
have seen ; but the fibres, if they do exist, are indistinct. 
There is a circumstance respecting one of the divisions of this 
nerve, that we have not observed in any portion of the first or 
second nerves, namely, that those fibres which go to the trumpet¬ 
like extremities of two of the semicircular canals, pass through 
a little knot or ganglion. I must not, at present, draw any con¬ 
clusion from this isolated fact, but I shall have hereafter to 
remind you of the ganglion on a part of the eighth nerve of peculiar 
sensation. The minute ramifications of this nerve are affected 
only by the vibrations of the air. 
Comparative Bulk of the Auditory Nerve. —When I come 
to treat of hearing, I shall have to describe many peculiarities, 
accounting for the greater acuteness of this sense in the quadruped 
than in the human being, arising from the external ear—the 
tympanum, the ossiculi, &c.; but at present I must only point 
out to you, in each of the brains before us, the greater bulk of 
the auditory nerve compared with that in man ; on the other 
hand, the adaptation of the human ear to the various modulations 
of tone, will be an interesting subject of inquiry. 
Comparison of these Nerves of peculiar Sensation. —Now then, 
gentlemen, let us for one moment recapitulate. We have ex¬ 
amined the three cerebral nerves of peculiar sensation —those of 
smell, sight, and hearing. We have traced each of them to that 
portion of the medullary matter at the centre of the brain, which 
answers to, or may be considered as a prolongation of, the surface 
of the spinal chord ;—we have further traced them to the centre 
