976 THE OSTEOLOGY AND MYOLOGY OF THE DEATH ADDER, 



SO by relaxing tbe internal pterygoid, and contracting the parieto- 

 palatine muscle and the spinalis group ; the latter pulling the 

 head upwards and backwards, while the parieto-palatine tends 

 to do the same for the palatine bone, which coming into contact 

 with the maxilla helps to raise that bone, and so aids in extricating 

 the fangs. When the fangs are once more free, the internal ptery- 

 goid contracts, and pulling back the transverse and pterygoid bones 

 depresses the fangs ; the parieto-palatine aiding in this by drawing 

 back the palatine. The fold of mucous membrane which surrounds 

 the fangs slips up to the base of the fangs when these are erected. 

 When depressed the mucous folds again regain their former 

 position. This is described in Crotalus as being brought about 

 by a slip of muscle from the pterygoid being attached to 

 the folds. In Acanthophis, however, the parieto-palatine sends 

 forward a slip which aids in this action. The chief element, how- 

 ever, appears to be some elastic fibres which are contained in the 

 membrane, and when the fangs are erected these are put on the 

 stretch ; but when the fangs are depressed the fibres assume their 

 former state, and so the membrane is brought back over the 

 fangs. 



In the non- venomous snakes the muscles attached to pterygoid 

 bones and palatine act so as to draw the bones forward or back- 

 wards, as the case may be. By this means the prey is drawn 

 gradually into the mouth. 



Movements of the Vertehrce. 



In regard to the movements of the spinal column, we have to 

 deal with no less than ten articular surfaces for each vertebra. 

 Two each on the zygosphene and zygantrum, two pre- and post- 

 zygapophyseal, and the ball and socket of the centrum. 



Taking two vertebrse that are articulated to one another, we see 

 the postzygapophysis of the anterior resting on the prezygapop- 

 hysis of the posterior, the zygosphene of the second with its 

 facets in the zygantrum of the first, and lastly, the ball of the 

 anterior resting in the socket of the posterior. 



