n8 THE BOOK OF NATURE STUDY 



when the animal bites. The poison gland, whose duct opens 

 at the base of the specialised teeth (or fangs) seems to be a 

 specialised salivary gland, equivalent to the parotid of mammals. 



When the snake opens its mouth to bite, the depressing of 

 the lower jaw automatically raises the maxillae bearing the fangs, 

 so that the fangs pass into a striking position. (A wooden 

 model can be made to show how one movable bone works on 

 another.) Moreover, the opening of the mouth and the erection 

 of the fang-bearing maxillae serve to squeeze the poison-gland. 

 When the tooth has a canal which opens near its tip the poison 

 must pass into the depths of the wound. When the teeth are 

 broken there are reserve fangs ready to take their place. We 

 find, in short, one effective adaptation after another ensuring 

 the snake's success. 



5. The sense organs of snakes are interesting. The peculiar 

 "stare," to which the credulous have ascribed a power of fascina- 

 tion, is partly due to the fact that the eyes are not in themselves 

 movable, and partly to the absence of the upper and lower eyelids 

 (seen as vestiges in the embryo). What is probably the equi- 

 valent of the third eyelid in many other animals forms a fixed 

 transparent blind across the eye, and is covered by a single 

 transparent watch-glass-like scale. Snakes have an internal ear 

 and can hear well, but there is no trace of ear-hole or drum or 

 Eustachian tube. The adder is not deaf, and it cannot stop its 

 ear. The sense of smell is well developed, and the long slender 

 bifid tongue, which has nothing to do with the poisoning, is a 

 restless organ of touch. 



Internally, too (though we shall not pursue the subject 

 further), there are many interesting adaptations; thus the right 

 lung is much larger than the left, as if there were not room for 

 both in the narrow body cavity ; the kidneys are not opposite 

 as in most animals, but one behind the other. 



A Grass Snake's Slough is sometimes found in the meadow 

 or a viper's on the moor. What is it, what is its peculiarity, 

 what is its significance ? The outermost horny layer covering 

 the scales dies away periodically, becomes worn, and is cast 

 off several times a year. It is replaced by another layer like 

 itself. It is not a " casting of the skin " that occurs, for this 



