GENERAL NOTES ON SNAKES. 631 



The bifid, mobile, retractile tongue is a specialised organ of 

 touch. In the mouth there is often a poison gland, which is a 

 specialised salivary gland. 



There are many peculiarities in the skeleton. The numerous 

 vertebra are all procoelous. 



The brain has only ten nerves. 



The heart is three- chambered, the ventricular septum being 

 incomplete, as in all Reptiles except Crocodilians. 



There is a transverse cloacal aperture. In the males a 

 double saccular and spiny copulatory organ is eversible from 

 the cloaca. 



Snakes are widely distributed, but are most abundant in 

 the tropics. 



ri 



FIG. 346. Snake's head. After Nuhn. 



<>., Poison fangs ; ., sheath of fang ; /., tongue; rl., muscles ot 

 tongue. 



General notes on snakes. Snakes, especially when 

 poisonous, are often brightly coloured. The scales on the 

 head form large plates, and those on the ventral surface are 

 transverse shields. In many cases there are odoriferous 

 glands near the cloacal aperture. 



The muscular system is very highly developed, and the 

 limbless serpent, Owen says, "can outclimb the monkey, 

 outswim the fish, outleap the zebra, outwrestle the athlete, 

 and crush the tiger." 



The vertebrae are very numerous, some pythons having 

 four hundred ; they are proccelous, and are distinguishable 

 only into a pre-caudal and caudal series. 



All the pre-caudal vertebrae except the first the atlas 

 have associated ribs, which are movably articulated, and 



