l82 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



from the dorsal side of the medulla, and just beyond its ganglion it 

 divides into two branches, one (pretrematic) going in front of the gill 

 cleft, the other (posttrematic) passing behind that opening. In the 

 seventh nerve the same conditions are repeated, the spiracle being but 

 a reduced gill cleft (fig. 143). In the same way the fifth nerve has 

 its pretrematic (maxillary) and posttrematic (mandibular) branches, 

 the mouth in this case being the opening above which the nerve 



Fig. 190. — Diagram of cranial nerves of lower vertebrate. Eye-muscle nerves 

 omitted; central nervous system dotted, fifth nerve represented as composed of two 

 nerves; lateralis nerves separated from the ninth and tenth nerves. I-X, cranial 

 nerves; 1-5, gill clefts; b, buccalis nerve; c, chorda tympani; g, geniculate ganglion; 

 h, hyoid nerve; i, intestinal (pneumogastric) nerve; j, jugal ganglion, /, lateral line 

 nerve of X; m, mouth; ind, mandibular nerve; mt, mentalis nerve; nix, maxillary nerve; 

 op, ophthalmicus profundus nerve; opV, opVII, superficial ophthalmic nerves of V 

 and VII; p, palatine nerve; po, posttrematic nerves; pr, pretrematic nerves; pt, petrosal 

 ganglion; s, semilunar (Gasserian) ganglion; sp, spiracle. 



divides. The tenth, which supplies all of the remaining gill clefts 

 may be regarded as a complex of as many nerves as there are clefts, 

 behind the first, though there is no embryological evidence of distinct 

 roots and ganglia. 



The ten (ichthyopsida) or twelve (amniotes) cranial nerves are 

 known by name and by number, the numbers being given in approxi- 

 mate accordance with the exits of the nerves from the brain. Be- 

 cause of their great importance the cranial nerves are described in 

 some detail. 



I. The olfactory nerve (Nervus olfactorius) extends from the ol- 

 factory lobe of the brain (rhinencephalon) to the sensory (olfactory) 

 epithelium of the nose. It differs from all other cranial nerves in that 

 it consists of prolongations (axons) of the sensory cells themselves, and 

 in having no ganglion separate from those cells. 



Close analysis shows that two different structures are frequently confused 

 under the term olfactory nerve. The true nerve consists of the fibres (olfactory 

 threads) which run back from the olfactory epithelium and terminate posteriorly 



