266 VERTEBRATE ANIMALS. 



periodically shed along with the rest of the external or epidermic 

 layer of the integument, and is again renewed. The pupil is round 

 in most serpents, but it forms a vertical slit or fissure in many ven- 

 omous Snakes and in the Boas. 



As regards the teeth, it is to be noticed that the snakes are not 

 in the habit of chewing their prey, but of swallowing it whole, and 

 the construction of their dental apjiaratus is in accordance with this 

 peculiarity. The lower jaw, as before said, articulates with the 

 skull by means of a quadrate bone (fig. 185), 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 peculiar 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 swallow- 



Fig. 191.— Skull of the Rattlesnake (after Dunici-ll and Bibroii). I One lialf of the lower 

 jaw united to the skull by the quadrate hone (q) ; m Upper jaw carrying the poison- 

 fang : p Serie.s of teeth upon the palate. 



ing. The teeth are simpiy fitted for seizing and holding 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 exist- 

 ing on the palate as well. In the venomous ."nakes, on the other hand, 

 the ordinary teeth are usually wanting upon the upper jaws, or are 

 few in number ; and these bones are themselves much reduced in 

 size. In place of the ordinary teeth, however, or in addition to 

 these, the upjier jaw.s carry the so-called "poison-fangs" (fig. 191). 

 In the Viperine Snakes these are a pair of long curved fangs, one 

 on each maxilla or ujjper ja^\bone, which, when not in use, are 

 pointed backwards, but can be raised at will by special muscles, by 

 the elevation of the movable maxillae. Each tooth is perforated by 

 a fine canal or tube, which opens by a distinct aperture at the point 



