STRUCTURE OF SERPENTS. 481 



When a venomous snake strikes, the mandible is lowered, 

 the distal end of the quadrate is thrust forward, this 

 pushes forward the pterygoid, the pterygo-palatine joint 

 is bent, the maxilla is rotated on its lachrymal joint, the 

 fangs borne by the maxilla are erected into a vertical posi- 

 tion, the poison gland is compressed by a muscle, and the 

 venom is forced through the fang. 



While there are no hints of anterior appendages, pythons, 

 boas, and some other snakes, have rudiments of a pelvis 

 and even small clawed structures which represent hind-legs. 



Some of the peculiarities in the internal organs of Ophidia, 

 may be connected with the elongated and narrow shape of 

 the body. Thus one lung, usually the left, is always smaller 

 than its neighbour, or only one is developed ; the liver 

 is much elongated ; the kidneys are not opposite one 

 another. 



The brain presents no remarkable peculiarities ; there are 

 only ten cranial nerves ; the sense of hearing is often slightly 

 developed, and there is no tympanic cavity ; the eyelids are 

 fused and transparent ; the bifid, mobile, retractile tongue 

 is a specialised organ of touch. 



The poison gland is a specialised salivary gland ; the 

 venom is useful in defence, and in killing the prey, which is 

 always swallowed whole. 



The heart is three-chambered, the ventricular septum being 

 incomplete, as in all other Reptiles except Crocodilians. 



There is a transverse cloacal aperture. In the males, a 

 double saccular and spiny copulatory organ is eversible from 

 the cloaca. 



The British adder (Pelias berus) is viviparous, and so are 

 a few others. The great majority are oviparous, but confine- 

 ment and abnormal conditions may make oviparous forms, 

 like the Boa constrictor and the British grass snake ( Tropi- 

 donotus natrix), viviparous. The female python incubates 

 the eggs. 



Many Ophidians become lethargic during extremes of 

 temperature, or after a heavy meal. 



Though most abundant in the Tropics, snakes occur in 

 most parts of the world. They are absent from many 

 islands ; thus there are none in New Zealand, and we all 

 know that there no snakes in Iceland. Most are terrestrial 



2 H 



