422 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



freely parallel to the sagittal plane. Right at the other (hinder) end of 

 the skull is another bone capable of a similar swinging movement. This 

 is the long slender quadrate bone to the lower end of which articulates 

 the lower jaw. A long bony strut, composed of pterygoid bone (behind) 

 and transverse bone (in front), connects the lower portion of the quad- 

 rate with the lower part of the maxilla, with the result that these two 

 bones are compelled to move in harmony like the two connecting pieces 

 in a pair of parallel rulers. When the mouth is closed the quadrate 

 swings back till nearly in line with the long axis of the head : the maxilla 

 swings back similarly so that the poison-fang is laid back under the 

 roof of the mouth. When, however, the mouth is opened the quadrate 

 swings forward and the maxilla moves in harmony with it and the poison- 

 fang is erected, ready to strike. 



The base of the poison-fang is enclosed in a special sheath of soft 

 tissue, the cavity of which opens freely into the opening at the base of 

 the fang. Into this same sheath opens the duct of the poison-gland. 

 This latter is the gland known as the parotid : it lies at the side of the 

 head posteriorly and its presence has much to do with giving the broad 

 appearance of the hind part of the head, contrasting with the compara- 

 tively narrow neck region just behind it, so characteristic of vipers. 

 The poison-gland lies in close contact with the muscles used in biting 

 and the contraction of these, pressing on the gland, squeezes out the 

 poisonous secretion and causes it to flow forwards into the tooth. 



The poison-fangs are peculiarly liable to injury and in correlation 

 with this there exists behind each fang a series of young fangs to replace 

 it. But it is not necessary that the fang should be broken or lost to 

 induce the replacement. In the European Adder during active life the 

 hums are replaced at fairly regular intervals. 



Although the broad head, sharply marked off from the neck, is 

 conspicuous in the Vipers it is not so in all poisonous snakes and is 

 therefore not to be trusted in forming a judgement as to whether or not 

 an unknown snake is poisonous. The Cobras (Naja) of Asia and Africa, 

 the Krait (Bungarus) of India, the Tiger Snake (Notechis) and the Death 

 Adder (Acanthophis) of Australia, the Coral Snake (Elaps) of America, 

 and the marine snakes, are well-known examples of poisonous snakes 

 \vhieh have not got this distinctive feature. 



As is the case with all the Amniota the visceral clefts, although they 

 duly make their appearance in the embryo, never develop gills. The 

 respiratory organs are lungs and these differ greatly in the degree of 

 their development in different reptiles. In many of the small lizards 

 they are no more complex than those of a frog the lung lining growing 



