324 HOWARD AYERS 



and cover these organ systems. As regards the nose, it is gener- 

 ally agreed that the olfactive organs are derived from a pair of 

 sensory organs, formed near the anterior end of the head which 

 very early sink below the surface of the skin of the snout, later 

 to become housed in a cavity called the nasal chamber. The de- 

 tails of this process and the structures involved have not been 

 definitely worked out and described. 



The following condensed account of some of the results of 

 my study of the vertebrate nose has for its object, making clear 

 the manner in which the human nose has come to be and also to 

 homologize the several structures entering into its formation. 

 The literature is extensive and regarding such structures as, e.g., 

 Jacobson's organ, the N. terminalis, and the vomeronasal nerve, 

 by no means harmonious either in statement of fact or interpre- 

 tation. No attempt will be made here to review the literature, 

 it being considered more important to lay the foundation for a 

 rationals tudy of the nose and nasal region of the vertebrate head. 



Beginning with the stage of head development presented by 

 Amphioxus, we pass to Ammocoetes, Petromyzon, Bdellostoma, 

 and Man. Already in the Marsipobranchs, the main features of 

 human nasal anatomy are l^tid down, since the nasal chamber 

 of Bdellostoma contains the terminal organs supplied by three 

 pairs of cranial nerves with the endings of the invading branches 

 of the trigeminus. 



1. AMPHIOXUS 



The anterior end of the head region of Amphioxus (figs. 1 and 

 12) is compressed from side to side and has the shape of a spear- 

 head, viewed either from the side or above. Included within it 

 we note the anterior end of the central nervous system with the 

 terminal paired but unseparated eye rudiments and the primitive 

 olfactive organs. This part of the nervous axis contains the 

 ventricular cavity and is the earliest stage known to us of the 

 vertebrate brain. Connected with the brain from before, back- 

 ward, are the following paired nerves (figs. 1, 2, 3, 4, 5, 6, 7, and 

 12) : 1. N. terminalis. 2. The N. opticus which does not extend 

 beyond the brain contour. 3. N. olfactorius. 4. N. septalis. 



