76 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



nostril to each olfactory sac, but in the selachians and ganoids 

 a fold of skin practically divides each nostril (external naris) 



FIG. 81. Relations of nasal organs to the cavity of the mouth; A, in fishes; 

 , in terrestrial vertebrates. l>, brain; z, internal nares ; n, external nares. 



into two. In many teleosts this is carried farther, and two dis- 

 tinct nostrils may occur on either side. These modifications 

 clearly are to permit a current of water over the olfactory 



epithelium (Fig. 80). 



In all vertebrates above fishes 

 both external and internal nares 

 (choana) are present, the latter open- 

 ing into the oral cavity. This condi- 

 tion is foreshadowed in the selachians, 

 where an oronasal groove leads back 

 from the external nares of either side 

 to the angles of the mouth. In the 

 higher vertebrates this groove be- 

 comes converted, during growth, into 

 a tube by the union of its edges. 1 

 In this way a respiratory tract is 

 formed on one side of the olfactory 

 surface, the posterior end of which 

 opens inside the cavity of the mouth. 

 In a similar way a naso-lachrymal 

 duct is formed leading from each eye 

 into the corresponding nasal passage. 

 In terrestrial vertebrates nasal glands 

 are frequently present in connection 

 with the nose, the secretion of which moistens the olfactory 

 epithelium. 



1 The process is modified in certain groups, where a solid cord of cells, instead of a 

 groove, is formed, the respiratory passage appearing later in the cord. 



FIG. 82. Head region of a 

 human embryo, after His, show- 

 ing the method of formation of 

 the respiratory nasal tract lead- 

 ing from the nostril, ;/, into the 

 oral cavity, and the naso-lach- 

 rymal duct leading from the eye, 

 e, into the nasal cavity. b, 

 rudimentary gill clefts ; k, hypo- 

 physial pocket ; /, lungs ; s, cer- 

 vical sinus. 



