588 



REPTILIA 



[CH. 



are no sternal ribs or sternum, but the dorsal ribs are elongated 

 and curved ventrally, and a Snake literally walks on the ends of 

 them: it is in a sense a vertebrate centipede. 



In the skull the chief 

 point to be noticed is the 

 extreme mobility of the 

 jaws. The jugal as well as 

 the quadratojugal have dis- 

 appeared, the pterygoids no 

 longer articulate with the 

 base of the skull, and the 

 quadrate itself is pushed 

 away from the cranium by 

 the squamosal 1 , which is a 

 rod-like bone. (Fig. 290). 

 The result of this arrange- 

 ment is that when the lower 

 jaw is pulled down, the 

 quadrate is quite free to 

 thrust the pterygoid for- 

 ward and push up the 

 maxilla by means of the 

 transverse bone ; that is 

 to say there is the same 

 mechanism as was described 

 in the Lizard, only more 

 easily set in motion and 

 capable of much more move- 

 ment. The halves of the 

 mandible, or lower jaw, are 

 connected by elastic fibres, 

 and thus they can be widely 

 separated. The result of this 

 is that a Snake has an enor- 

 mous gape and can swallow 

 prey almost as large as itself. 

 Snakes of quite moderate 

 size dispose of frogs, birds, etc. The large Pythons of India can 

 crush an animal larger than a half-grown sheep into a shapeless 



1 This so-called squamosal corresponds to the upper branch of the squamosal 

 of Lacerta, and represents the tabular bone. 



24 



FIG. 291. Diagram of arterial arches of 

 Snake viewed from the ventral aspect. 



i, n, in, iv, v, vi. First to sixth arterial 

 arches. 12. Tracheal (ventral carotid). 

 13. Common carotid (dorsal carotid). 15. 

 Eight systemic arch. 16. Left systemic 

 arch. 17. Dorsal aorta. 19. Pul- 

 monary. 24. Coeliac. 



