DIYISIONS OP EEPTILIA. 



243 



(Fig. It'll), and this in turn is movably jointed to the cranium. 

 The two halves of the lower jaw are also merely united 

 loosely in front by ligaments and muscles. In consequence 

 of this pectdiar arrangement of parts, the serpents have the 

 power of opening the mouth to an extraordinary width, and 

 they can perform the most astonishing feats in the way of 

 swallowing. The teeth are simply fitted for seizing and hold- 

 ing the prey, but not in any way for chewing or dividing it. 

 In the harmless snakes, the teeth are in the form of solid 

 cones, which are arranged in rows round the whole of the- 

 upper and lower jaws, a double row existing on the palate 

 as well. In the venomous snakes, on the other hand, the 

 ordinary teeth are usually wanting upon the upper jaws, and 

 these bones are themselves much reduced in size. In place 

 of the ordinary teeth, however, the upper jaws carry the so- 

 called " poison-fangs " (Fig 131, B). These are a pair of long, 



Fig. 121.— a. Diagrammatic Section of the Eye of a Viper (after Cloquet). a Eyeball; J 

 Optic nerve; c Chamber into which the tears are poured; d Epidermic layer covering 

 the eye. B, Head of the common Viper (after Bell), showing the poison-fangs. 



curved fangs, one on each maxiUa or upper jawbone, which, 

 when not in use, are pointed backward, and concealed in a 

 fold of the gum, but can be raised at will by special muscles. 

 Each tooth is perforated by a fine canal or tube, which opens 

 by a distinct aperture at the point of the fang, and is connect- 

 ed with the duct of the " poison-gland." Tiis is a gland, 

 situated under and behind the eye, secreting the poisonous 

 fluid which renders the bites of these snakes dangerous or 

 fatal. When the serpent strikes at any animal, the poison is 

 forced through the poison-fang into the wound, partly by the 

 contractions of the muscular walls of the gland, and partly by 

 the compressive action of the muscles of the jaws. In some 

 other snakes, several of which are not certainly known to be 



