portion bears a deep cavity for the insertion of the temporal 

 muscles ; in the vipers a thin plate rises on the inner side of 

 this cavity. Close to its posterior end the mandible articulates 

 with the tympanic bone, 13. In the harmless snakes this is 

 short and stout, in the venomous snakes it becomes long 

 and slender. Its upper extremity is suspended from the 

 mastoid, a partly detached bone of the skull. 



Owing to the loose connection of the mandibles at the 

 chin, and to there being three joints connecting successively 

 the mandible, the tympanic and the mastoid to the skull, 

 the lower jaws have an extensive range of motion both per- 

 pendicularly and horizontally, and are also to a certain 

 extent independent of one another. The maxillary and 

 palatine of either side being also loosely connected to the 

 base of the skull are capable of being pushed forward or 

 retracted by the pterygoid bones, themselves loosely attached 

 posteriorly. Each of the four half-jaws being then capable 

 of independent motion a snake is able to advance them one 

 at a time, the prey he is swallowing being securely held by 

 the other three. Also, the action of the pterygoid bones 

 being to push forward the maxiUary and palatine, if 

 either of these be fixed at one point, the to and fro motion 

 will become converted into a circular motion with the point 

 of attachment as a centre. This is the principle on which 

 the poison-fang is erected in venomous snakes. 



CHAPTER II. — The Mouth and Teeth of harmless 



Snakes. 



The gape of the mouth is very considerable and, owing 

 to the multiplication of joints between the skull and the 

 mandibles, the upper and lower jaws can be separated until 

 they form nearly a straight line. The mouth is never 

 opened except for the purpose of seizing prey or in defence 

 or sometimes in yawning, after food or drink ; a chink in 



