364 REPORT OF NATIONAL MTISEFM, 189:}. 



Tlie iiHine rcfcis to tlic deep pit or liole found in the Rattlesnakes 

 and their nearest relations on the side of the faee between the nostril 

 and the eye, and well shown in iig 0. This eavity sinks dee}) into the 

 maxillary bone and represents a " blind" sac lined with epidermis and 

 is not connected with any of the other cavities or organs in the head 

 by any inside opening' or canal. There is nothing similar to be found 

 in any known reptiles outside of this family, if we except the labial 

 pits in the i)ythons and boas, nor is there in any other class of animals. 

 When the earlier zoologists came to examine this peculiar structure 

 they, of course, tried to compare and identify it Avith other organs 

 already known, some hinting at the closed nostrils of the fishes, while 

 others pointed out its similarity, in position at least, to the so-called 

 " tear-sacs '' of the deer. It was even suggested that in view of the close 

 ai)proximatiou of the pit to tlie poison apparatus it might have for 

 object the admission of air to act in some unknown way u])on the secre- 

 tion of the venom glands, thus rendering the poison more ])owerful. 



Fig. 9. 



UliAU OP CKOTALUS, FROM SIDE.. 



I InCralabiiils ; Horeal; Z' lower loreal ; Z^ upper loreal ; 7i nasal; it ' anterior uasal ; ji- posterior nasal -, 

 /; ]ireo( iilar; r rostral ; s snpralabials ; s' 1st supralabial; s^Sd supralalii;il, etc. : so stipraoeiilar 



It was phiin, however, even to those who proposed these explanations, 

 that they were not the true solution of the question, and most authors 

 were satisfied with a reference to the pits as "mysterious." 



In the meantime naturalists have become compelled to assume the 

 existence of a "sixth sense'' in various animals, tor which they had 

 discovered special sense organs, such as, the lateral line in fishes. 



It was (piite natural, then, that Prof. Leydig should come to the con- 

 (;lusion that the pit of tlie Crotalidw is the organ of a sixth sense, when 

 upon a microscopic examination of the pit's lining he found it supplied 

 with a thick nerve, ending in a way the only analogue of which is found 

 in the retina of the eye or the labyrinth of the ear. 



Leydig's material was in some respects defective, for he had only 

 specimens preserved in alcohol, and in his valuable memoir* on the sub- 

 ject twenty-five years ago he distinctly encouraged the North American 

 naturalists to take the matter up and continue his in vestigati(ms on fresh 

 specimens, but, as far as I know, nobody has as yet done so. It may 

 therelbre not be out of place to give a brief synopsis of his observations. 



*Nova Acta Acad. Caes. Leopold. Nat. Cnrios., xxxiv, 1868, No. 5, pp. 89-96> pi. 

 iv, figs. 28-32. 



