578 REPTILES. 



ing is often slightly developed, and there is no tympanic 

 cavity. 



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. 



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 



rl 



Fig. 251. — Snake's head. — After Nuhn. 



dv., Poison fangs ; &., sheath of fang ; -/., tongue ; rl. t muscles of 

 tongue. 



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. 



AH the pre-caudal vertebra except the first — the atlas — 

 have associated ribs, which are movably articulated, and 

 used as limbs in locomotion. In the caudal region the 

 transverse processes, which are elsewhere very small, take 

 the place of ribs. 



