NERVOUS SYSTEM 



881 



passes in front of the gill cleft, and the other, the post-trematic, which 

 passes behind that opening. The seventh nerve is arranged in a manner 

 quite similar to that of the ninth, the spiracle being a reduced gill cleft. 

 The fifth nerve, which is also divided in the same way, has its post- 

 trematic, called the maxillary, and the pre-trematic branch, the mandibu- 

 lar, the mouth forming the opening about which this nerve divides. The 



Fifir. 494. 



Diagram showing: cranial nerves of a cat with the lower 

 jaw reflected. Il-XII, cranial nerves ; ct., chorda tympani ; d, 

 dentary nerve ; g, Gasserian ganglion ; to., infraorbital nerve ; 

 I, lingual nerve ; li.la., laryngeus inferior and superior ; md, 

 mandibularis nerve ; nix., maxillaris nerve ; o., ophthalmic nerve ; 

 ., tongue. (From Kingsley after Mivart). 



tenth or vagus nerve supplies all of the remaining gill clefts and is there- 

 fore supposed to be complex, composed of as many nerves as there are 

 clefts behind the first; however, there is no embryological evidence of 

 distinct roots and ganglia. 



The cranial nerves are of considerable importance in all physiologi- 

 cal, neurological and pathological work, so it is important that they be 

 thoroughly studied. 



I. The Olfactory Nerve connects the olfactory lobe of the brain 

 with the sensory epithelium of the nose. It is different from all other 

 cranial nerves in that it consists of many tiny prolongations of the 

 sensory cells themselves, and in having no ganglion separate from these 

 cells. The true olfactory nerve consists of these tiny fibers or threads, 

 extending from the olfactory epithelium in the nose to the so-called 

 mitral cells in the olfactory lobe. The places where the dendrites of 

 the mitral cells meet with the terminations of the olfactory fibers from 

 the olfactory epithelium are known as glomeruli. In the dogfish, for 

 example, and in many fish, snakes, lizards, and mammals, the true olfac- 



