THE OPHIDIA. 25 



slanting backwards, so that it is impossible for an animal held by 

 them to retreat. All snakes are furnished with palate teeth, two 

 rows extending backwards and all curved and slanting in the 

 same manner. The upper and lower jaws are also well supplied, 

 so that we may call them six rows of teeth. In a non- venomous 

 snake their position may be represented as in a (fig. 7). In 

 the viper the upper jaw is extremely short, and furnished with 

 only the poison fang (c) ; a cobra and some other venomous 

 colubrines haVe one or two simple teeth in addition to the 

 fangs (fr). All these finely pointed and delicate teeth, having a 

 good deal of strain upon them during the process of feeding 

 (not for mastication, but to retain bulky prey), are liable to 

 get broken off or to come out easily ; but buried in the gum are 

 always a plentiful supply of young teeth ready to replace them. 

 Snakes can renew their teeth throughout life, or until very old 

 age or infirmities overtake them. There was a python of at least 

 thirty years old at the Zoological Gardens who had ceased to 

 renew its teeth. They were by degrees lost and not replaced, and 

 the poor old snake had great difficulty in feeding, but, as a rule, 

 nature has beneficently supplied them with these all-important 

 assistants towards supporting life. 



We have seen that a snake can exsert, withdraw, and safely 

 enclose its tongue ; that its head can expand out of all proportion, 

 and regain its form without inconvenience ; that it can manage 

 its trachea so as to breathe when its mouth and throat are gorged ; 

 and now we shall see that it can move its ribs, every single pair 

 of them, as easily as we can move our arms or fingers. The ribs 

 are elaborately articulated with the spinal column, as described on 

 p. 19, and expand for the reception of bulky food. They are also 

 used as legs, and upon the tips of them the snake walks or glides. In 

 the larger snakes it is not difficult to watch the action, like 

 undulations, beneath the skin. Three or more sets of ribs are in 

 motion at the same time, but in regular and wave-like succession, 

 the "wave" as it were, started with the anterior ribs, passing 

 down through the entire length of the snake, and succeeded by 

 another and then another set, as the first movement passes on ; 

 just as you see a long streamer or a curtain-fringe moved by the 

 wind, wave after wave following in succession, so that you can 

 scarcely define where each undulation begins, all moving on 

 harmoniously. When a serpent is actually progressing, every 

 single pair of ribs is, of course, in action : but should only one 

 part of the hody be stirring, the ribs at that part only may be 

 active. To each pair of ribs one of the broad under scales 

 (ventral scuta) is attached by muscles, so that these also move 



