54 STRUCTURE OF OPHIDIANS. 



remarkable : the anterior part of the body of each vertebra pre- 

 sents a rounded semi-spherical tubercle which is received in a 

 corresponding cavity on the posterior face of the vertebra next 

 to it. This mode of articulation explains the movements of the 

 body of these animals, which, in general, are executed laterally, 

 and not from above downwards; in fact, the spinous processes 

 which prevail along the back are ordinarily so disposed as to 

 prevent the vertebral column from bending very much in this 

 direction. The ribs surround a large part of the circumference 

 of the trunk, and are wanting only on the caudal vertebrae. The 

 first pair are smaller than the others ; but they commence at the 

 head, so that these animals have no neck. We count in some 

 instances as many as 250 pairs. 



43. Serpents are essentially carnivorous ; they can endure 

 abstinence for a long time ; but, in general, when the opportunity 

 occurs, they gorge the stomach with such a quantity of food, 

 that, during digestion, they remain in a state of greater or less 

 torpor : they do not chew their food ; but their mouth is armed 

 with hooked teeth, suited for retaining their prey. Their tongue 

 is very extensible, and terminates in two long, semi-cartilaginous, 

 and very moveable filaments. The digestive canal is very short, 

 and the stomach is merely a slight dilatation of this tube. 



44. The circulation is carried on in the same manner as in 

 Tortoises and Saurians, except Crocodiles. The heart is com- 

 posed of two auricles and a single ventricle, incompletely divided 

 into two cavities, from each one of which arises an aorta, which 

 joins its fellow behind this organ. The lungs are very unequal 

 in size, and generally one of the two is entirely atrophied, while 

 the other is very large, and prolonged into the abdomen, above 

 and beyond the stomach and liver : it is in the form of a large 

 membranous sack, in the interior of which are found great poly- 

 gonal cells. The air is renewed in them by the motions of the 

 ribs and abdominal muscles, nearly in the same way as it is in 

 saurians and birds. Probably, it is in part owing to the position 

 of the lungs that serpents become torpid after a copious repast ; 

 for, every time they swallow a prey of large size, this organ must 

 be compressed, and the pulmonary circulation impeded. 



45. The structure of the head varies in these animals. Some- 

 times the lower jaw, the two branches of which are solidly united 

 in front, is supported on a tympanic bone, which is itself immedi- 



43. Upon what do Serpents feed ? Do they chew their food ? What is 

 the character of their digestive organs ? 



44. What are the peculiarities of the circulation in Serpents? What is the 

 character of their respiratory apparatus? Why are Serpents torpid after a 

 full meal ? 



45. IB the structure of the head the same in all these animals ? 



