338 HOWARD AYERS 



brain, in fact, from the territory of the trigeminus nerve; we there- 

 fore find that the advanced outposts of the brain, the most 

 anterior sense organs of the body surface, in all forms above 

 Amphioxus are tactile sense organs of simple or complex struc- 

 ture belonging to the trigeminus as distinguished from the 

 earlier group of chemical sense organs, belonging to the ter- 

 minal, olfactory, and septal nerves which form the anterior sen- 

 sory outposts in Amphioxus. This change in the character of 

 the sensory outposts is of course a result of forward overgrowth 

 of the trigeminus mechanisms. We also find that the tri- 

 geminus structure has not only surrounded and housed in this 

 group of chemical sense organs, but has also invaded the origi- 

 nal olfactory and terminal sensory territories supplied by the 

 NN. terminalis, olfactorius, and septalis, and here performs some 

 function of a tactile sort. Along with the housing of the chemi- 

 cal sense organs there has been built up in the long series of verte- 

 brate forms a large variety of control systems of valves, doors, 

 guide folds for the control of water and air currents, their admis- 

 sion, guidance, and expulsion varying in different animals as the 

 case may be. The emergence of vertebrates from water breath- 

 ing to air breathing has not affected the physical conditions of 

 the functioning of the chemical sense organs, for they still are 

 kept wet and pick up their stimuli out of a liquid medium, 

 thanks to the moisture supplied by the 'mucosa.' With the 

 exception of the jaw apparatus and related parts, no single 

 change of, or addition to head structures has caused greater 

 changes in contour or anatomical detail than the housing of the 

 chemical sense organs. Owing to the failure to recognize the 

 presence of both the terminal and septal nerves, the first arising 

 ventrad of the olfactorius, while the second arises dorsad of the 

 olfactorius, much confusion exists in accounts of the so-called 

 nervus terminalis. In all vertebrates yet examined, one of these 

 nerves is found to be present, which may arise near the dorsal 

 surface of the brain or near the ventral surface. In many forms 

 both pairs of nerves have been found. Further investigation 

 will doubtless show that both nerves are present in all vertebrates. 

 In order to make final decision, it will be necessary to trace the 



