COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



cerned differ considerably in structure and relations. The olfactory 

 epithelium is always restricted to one or two patches at the anterior end 

 of the head and differs from the taste buds in histological structure. 

 Both sensory and supporting cells of the olfactory organs are variously 

 constituted. The supporting cells are the stouter, some being ciliated, 

 some muciparous at their free ends. The sense cells (fig. 176, C) are 

 thread-like or rod-like, being greatly expanded around the spherical 

 nucleus, while the basal end of each contracts to a nerve fibre which 

 extends back to the olfactory tract (p. 168), where the dendrites, inter- 

 lacing with those of the olfactory lobe, form the glomeruli. In the 

 higher vertebrates a third kind of cells, the basal cells, occur at the base 

 of the olfactory epithelium. 



The olfactory epithelium arises as part 

 of the surface ectoderm of the top of the 

 head, but with growth it changes its position. 

 For protection it sinks beneath the surface as 

 an olfactory sac, connected with the external 

 world by (usually) a pair of openings, the ex- 

 ternal nares. The growth of the dorsal side 

 of the head carries the nares toward the tip 

 of the snout and, in the elasmobranchs, to 

 the ventral side of the head. 



The accessory parts of the olfactory 

 , organs are the skeletal nasal capsules (p. 



FIG. 189. Nasal organ of , 



caecilian (JE^cwwwf), after Sara- 62), which are always present; in the tetra- 



podous forms glands to keep the epithelium 



lateral cavity; mp, middle pas- moist, and the organ of Tacobson. The in- 

 sage; os, olfactory sac. . 



volution of the nasal sacs necessitates some 



mechanism for bringing the external medium (water or air) to the sen- 

 sory cells. These will be described in connection with the several 

 groups below. The organ of Jacobson is a kind of accessory 

 olfactory organ, first appearing in the amphibia, supplied by the first 

 and fifth nerves and apparently serving to test the character of the food 

 while in the mouth. The position of the organ near the internal 

 nostrils lends probability to this view of the function. 



The cyclostomes differ markedly from the other vertebrates in their olfactory 

 organs. The unpaired area of olfactory epithelium develops in the region of the 

 anterior neuropore (p. 12) and becomes involved with the involution for the 

 hypophysis (fig. 190) so that there is but a single external opening, serving for both 



