PISCES. 



995 



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. The optic nerves (Jig- 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 (Jig. 

 528, n). In the Skates, however, a commissure 

 exists similar to that which is met with in the 

 higher Vertebrata. In some Fishes each optic 

 nerve 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 (Jig. 529, 3) 

 arises from the medulla oblongata in the track 

 of the pyramidal bodies, and is distributed, as 

 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 (Jig. 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 of the eye. 



5. The fifth pair of nerves (fig. 529, 5) 

 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 lemporale 

 and gives branches to the temporal muscle, 



Fig. 529. 



Brain and cerebral nerves of Cod-fish ( Gadus mor- 

 rhua). (After Swan.) 



a, olfactory lobes ; b, hollow or cerebral lobes; 

 c, cerebellum; d, medulla oblongata ; e, olfactory 

 apparatus ; f, eye-ball ; g, superior oblique muscle ; 

 h, external rectus; the numbers I, 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 branchiostea;ous membrane. 6. The fifth 

 pair almost invariably gives off a branch svhich 

 mounts to the upper part of the cranium, and 

 joining a branch of the eighth pair issues 



3 s 2 



