SENSE ORGANS 75 



often quite to the length of the head, and furnished 

 with many sensory corpuscules. It is darted and 

 vibrated on the least excitement, and is usually looked 

 upon by the ignorant as a " sting." In most snakes 

 it is much pigmented, dark brown or black ; in a few 

 it is flesh-coloured or bright red. The tongue is 

 entirely retractile into a sheath below the glottis and 

 opening in front of it ; it is always withdrawn into 

 the sheath when the snake bites or feeds. 



Other organs, which, in the absence of a satisfactory 

 explanation of their use, have been termed ''organs of 

 a sixth sense," reside in the head-shields and scales of 

 many snakes, and in the deep pits on the sides of the 

 head which are characteristic of various Boidse and 

 a few Colubridae. 



Scales often show, near their posterior extremity, 

 one or two small light spots or impressions, caused 

 by a thinning of the epidermis, which have been 

 called "apical pits"; they appear to coincide with the 

 terminations of nerve fibres extending along the 

 epidermal folds of the skin. Similar organs some- 

 times form series on the borders of some of the head- 

 shields, this being particularly noticeable in the 

 Typhlopidae. 



The large and deep pit situated between the nostril 

 and the eye (loreal pit) in the Crotaline Viperidse 

 — whence the name Pit-vipers, or that of " cuatro 

 naricas" which is bestowed on them by the Spaniards 

 of Mexico — is divided into two chambers: an outer 



