XIII 



PHYH^M CH(^RDATA 



109 



ovor almost the whole surface of the body. In hii^her Craniatcs, 

 with the excei)ti<)ii of Birds, they are chiefly conhned to the epi- 



Kii 



. 7S2.— A, voi-tical sci-tiuu uf oiio of the iMiiillu- of tlic tongue of a Mammal. >', «"')- 

 mucosa ; c. eiiitlieliuin ; n. iicrve-tibres ; t. taste-buds. B, two taste-buds. c. covcviiig cells 

 shown in lower bud; <l, sub-mncosa ; c. einthclium of tongue; m, sensory in-ocesses ; n. 

 internal sensory cells shown in upper I)ud. (From Foster and Shore's Physiolouy.) 



thelium of the tongue and soft palate, and are supplied by the 

 gustatory branches of the trigeminal, facial, and glos.sopharyngeal. 

 The olfactory organ is typically a sac-like invagination of the 

 skin of the snout, anterior to the mouth, and communicating with 

 the exterior by an aperture, the external 

 nostril. It is paired in all Craniata, ex- 

 cept Cyclostomes, in which there is a 

 single olfiictory sac, supplied, however, by 

 paired olfactory nerves. The sac is lined 

 by the olfactory mucous membrane or 

 Schnciderian membrane, the epithelium of 

 which contains peculiar, elongated sensory 

 cells (Fig. 783), their free ends often pro- 

 duced into hair-like processes. In the 

 Dipnoi and all higher groups the posterior 

 end of each sac communicates with the 

 cavity of the mouth by an aperture called 

 the posterior nostril, and an analogous 

 communication occurs in the case of the 

 unpaired organ of the Hags (vide ivfra). 



In many air-breathing Vertebrates there 

 is formed an offshoot from the olfactory 

 organ, which, becoming separated, forms 

 a distinct sac lined with olfactory epi- 

 thelium and opening into the month. This 

 is Jacobson's organ : it is supplied by the olfactory and trigeminal 

 nerves. 



The paired eye is a more or le.ss globular structure, lying in 

 the orbit, and covered externally by a thick coat of cartilage or of 



Fig. 7S3.— Epithelial cells of 

 olfactorj- nuicous membrane. 

 A, of Lamprey; B, of 

 Salamander. M. inter- 

 stitial cells ; Ji, olfactory 

 cells. (From W'iedersbeini's 

 Verteln-ata.) 



