PISCES. 
and vary much in their size and composition in 
different Fishes. For the most part they are 
simple, but sometimes double or triple; and 
occasionally they are composed of numerous 
filaments united into fasciculi. Generally they 
present a ganglionic enlargement just after their 
origin from the brain, or immediately before 
their termination in the olfactory organ, to the 
plicated folds of the lined membrane of which 
they are ultimately distributed. 
2. Theoptic nerves (fig. 529, 2) arise from 
the second pair of the great cerebral masses, 
which from this circumstance have been called 
the optic lobes of the brain. Shortly after their 
origin the two nerves cross each other, but in 
the generality of osseous Fishes this is effected 
without any union of substance, the two nerves 
being simply united by cellular tissue (fg. 
528,”). In the Skates, however, a commissure 
exists similar to that which is met with in the 
higher Vertebrata. In some Fishes each optic 
herve consists of a broad flat nervous band 
folded upon itself like a fan, and enclosed in a 
tube of neurilemma derived from the dura 
mater; but in others their structure resembles 
that which exists generally in the higher ani- 
mals. 
3. The third pair or motor oculi (fig. 529, 3) 
arises from the medulla oblongata in the track 
of the pyramidal bodies, and is distributed, as 
995 
in other Vertebrata, to all the muscles of the >| 
eye, with the exception of the superior oblique 
and external rectus. It likewise furnishes cili- 
ary nerves, but no ophthalmic ganglion has yet 
been discovered in the class before us. 
4. The fourth pair (fig. 529, 4) arises just 
behind the posterior point of the optic lobe from 
the roof of the ventricle, and terminates in the 
superior oblique muscle cf the eye. 
5. The fifth pair of nerves (fig. 529, 5) G 
arises from the sides of the fourth ventricle 
near the base of the cerebellum. It issues from 
the cranium through a foramen in the great alar 
bone, and is distributed as follows:—1. It gives 
off an ophthalmic branch which runs along 
the roof of the orbit, and passing on towards 
the nose is distributed to the adjacent parts of 
the face as far as the snout and intermaxillary 
bone. 2. A superior maxillary branch, which 
passes under the eye to be distributed to the 
cheek and to the superior maxilla; it likewise 
sends a branch towards the nostrils and anas- 
tomoses with the pterygo-palatine nerve. 3. 
An inferior maxillary branch, which is fre- 
quently only a division of the preceding: this 
gives filaments to the posterior part of the 
palate, and passes on to the inferior maxilla 
and its dental canal. Frequently the palatine 
filaments proceed from a special branch. 4, A 
pterygo-palatine branch, which runs forwards, 
crossing the floor of the orbit beneath the 
muscles of the eyeball, follows the course of 
the vomer, and passes beneath this bone and 
the os palati to terminate at the end of the 
muzzle, where it is frequently joined by re- 
markable anastomoses with the superior maxil- 
lary branch. 5. An opercular branch, which 
passes through a canal in the os temporale 
and gives branches to the temporal muscle, 
Brain and cerebral nerves of Cod.fish ( Gadus mor- 
rhua), (After Swan. ) 
a, olfactory lobes; 6, hollow or cerebral lobes ; 
c, cerebellum; d, medulla oblongata ; e, olfactory 
apparatus ; f, eye-ball ; g, superior oblique muscle ; 
h, external rectus; the numbers lL, 2,3, 4, &c., in- 
dicate the corresponding cerebral nerves. 
to the cheek, to the muscles of the operculum, 
and to the operculum itself; it then penetrates 
internally to join with branches of the inferior 
maxillary divisions and to supply filaments to 
the branchiostegous membrane. 6. The fifth 
pair almost invariably gives off a branch which 
mounts to the upper part of the cranium, and 
joining a branch of the eighth issues 
38 
