I 



OLFACTORY ORGANS 205 



the dendrites, interlacing 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 epithehum 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 external 

 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. 

 69), which are always present; in the tetra- 

 podous forms glands to keep the epithelium 

 moist, and the organ of Jacobson. The in- 

 volution of the nasal sacs necessitates some 

 mechanism for bringing the external medium 

 (water or air) to the sensory cells. These 

 will be described in connexion with the 

 several groups below. The organ of 

 Jacobson is a kind of accessory olfactory fig. 211.— Nasal organ of 

 organ, first appearing in the amphibia, cajcilian(£^imMf») after Sara- 



1- J L .1- ^- . 1 /-r.i ^'"^- cA, choana; JO, organ of 



SUpphed by the first and fifth nerves and Jacobson; W, lacrimal duct; //>, 



apparently serving to test the character of Sge7L%t"c=to"^'sTc'''''^ 

 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 carried in with the involution for the 

 hypophysis (fig. 212) so that there is but a single external opening, serving for 

 both olfactory organ and hypophysis. Hence cyclostomes, having but a single 

 nostril, are called monorhinal, in comparison with all other vertebrates which 

 have two nostrils (amphirhinal). The median opening or naris of the cyclo- 

 stomes connects with a naro-hypophysial duct, on the upper, posterior wall of 

 which is the olfactory sac, formed of pairs of lateral folds (fig. 213) covered 

 with the olfactory epithelium and supplied by a pair of olfactory nerves. The 

 lower part of the duct, now purely hypophysial, descends to the hypophysis on 

 the ventral side of the brain, where it either ends bhndly (petromyzons) or 

 opens into the dorsal part of the oral cavity (myxinoids). In the latter group 

 the olfactory organ is surrounded by a complicated nasal capsule of enormous 

 size (fig. 171). 



