340 HOWARD AYERS 



6. COMMENTS ON FUNCTION 



We know almost nothing of the function of the terminal organ, 

 of Jacobson's organ, of the hypophysis. We are perhaps war- 

 ranted in assuming that the terminal, olfactory, and septal nerves 

 have to do with special chemical senses, of which above Amphi- 

 oxus the olfactory sense plays the predominant role. Next in 

 importance stands the terminal sense organ, which we find 

 from the fishes on, as the organ of Jacobson, a paired sense organ 

 of the nasal chamber which appears reduced in importance as 

 compared with the olfactory organ, although in the Ophidia the 

 reverse appears to be the case. The functions of the nerves in 

 the nasal chamber may be divided as follows: 



f Olfactory nerve Testing alimentary foods 



Chemical sense < Terminal nerve \ m i- • ^ *■ j 



[Septal nerve / ^^'^'""^ respiratory foods 



[Testing for solid bodies in 



m .-1 rr. • • 1 ] the respiratory currents and 



Tactile sense Trigeminal nerve < . ^, "^ , ^, 



I sensing the pressure and the 



[ current flow 



Although the chemical sense organs have been housed in the 

 nasal chamber, they have, so far as the structure of the sensory 

 surfaces are concerned, remained in a primitive state. These sense 

 organs have not developed accessory parts in such degree as 

 have the eye and ear. In the case of the eye, the vitreous 

 body, lens, cornea, lids, etc., have been added to increase the 

 precision or enlarge the range of its functions, there has been 

 added to its light-perceiving function the optical reactions. 

 In the case of the ear, there has been a parallel evolution of the 

 primitive function of wave-motion perception by the addition of 

 tone perception, with the cochlea as its anatomical expression. 

 In both cases there has been a progressive increase in the number 

 of protective structures as well as of parts serving the increase 

 of precision and enlarged range of function. In the case of the 

 chemical sense organs of the nasal chamber there has not been 

 such considerable increase of subsidiary parts or perfection of the 

 sensory structures, and they therefore remain organs whose stim- 



