164 ON THE ORGAN OF HEARING 
lous plexus, in order to rouse or call into action the sen- 
sitive qualities of the termination of its fasciculi. 
In the thornback, there is a beautiful long slender nerve 
given off at the beginning of the plexus, which runs up- 
wards, and is inserted into the substance of the great sac, 
near the opening of the posterior canal into the sac. 
The above is the most frequent manner in which the 
branches are distributed in forming the plexus ; but, in a 
very old shark, I met with the following arrangement, 
which, as it had a beautiful appearance, and was rather 
uncommon, at least to my experience, I shall take the li- 
berty to describe in as few words as possible. 
The sabulous nerve, on its entrance into the vestibule, 
divided into four branches, three of which were extremely 
slender, while the fourth (anterior to them) was larger 
than the three put together. This large branch ran out- 
wards, forwards, and then backwards, so as to form an ob- 
long semicircle of six or seven lines in diameter, where it 
united with the other three slender branches which ran 
obliquely outwards, and formed a line parallel to the dia- 
meter of the oblong semicircle. These three slender 
branches passed into the fossa, and formed the cornu. The 
large branch, which might be termed the Nervus semicir- 
cularis, from its curvature, and v/hich is precisely the nervus 
communicans of our former description, gave off many 
branches. Those that were on the inside, or posterior to 
the nerve, proceeded obliquely towards the long slender 
branches, and, in their course, gave off numerous slender 
filaments, that united with the other branches, so as to 
form a beautiful plexus, similar to the spherical plexus, but 
more extensive. 
The plexus was of an oblong semicircular figure, and 
connected so as to resemble a net, the meshes being formed 
by the branches and filaments, as above, while, at the time. 
