550 



THE NATURE BOOK 



m 



upper portions 



ever, are higlily specialised. The \'erte- 

 brae, concave beforehand convex behind, 

 form a succession of ball and socket joints, 

 and thus combine strength with mobility. 

 There is a limit to the mobility, however, 

 which is secured by the interlocking 

 certain positions of the 

 of successive vertebrae. 



Each body vertebra 

 has a pair of ribs at- 

 tached to it. and these 

 ribs, the distance of which 

 from one another admits 

 of considerable expansion, 

 are fixed by their distal 

 ends to the broad over- 

 lapping plates w h i c h 

 cover the under portion 

 of a snake's body. In 

 a moving snake a wa%'e 

 passes from head to tail 

 as the sharp hind edges 

 of these plates succes- 

 sively engage and quit 

 the ground. It is, of 

 course, essential for this 

 movement that a con- 

 siderable portion of the 

 snake's body should be 

 in contact with the 

 ground surface, and the 

 snake obtains the path 

 of least resistance by 

 throwing his body into 

 horizontal convolutions 

 and ghding round, rather than surmount- 

 ing obstacles. 



In explanation of the conventional 

 drawing which represents snakes as mo\'ing 

 in vertical rather than horizontal con- 

 volutions. I would hazard the following 

 conjecture. It is well known that the 

 earliest hne drawings were in strict 

 profile. Ancient art had no conception 

 of fore-shortening. Suppose then (and 

 this must frequently ha\'e occurred under, 

 say, Ramc^es XXXII.) that an artist 

 \yas commissioned to draw a snake. His 

 first impulse would be to draw it, as he 

 drew everything else, sidewa}'s. With 

 a hve model this could only result in a 

 straight and stumpy snake. He would 

 woiTy abcmt the wrongness of his picture, 

 and the critics would worrv him, but at 



UPPER SURFACE OF AN ADDER'S 

 HEAD MAGNIFIED. 



The principal plates are the frontal (F), 

 the parietals {P P), and the supra- 

 oculars (S 6). It should be noted 

 that the supra-oculars are separ- 

 ated by small scales from the 

 frontal. In the Grass Snake and 

 Smooth Snake, the supra-oculars 

 and frontal are usually contiguous. 



Why should he not cease to grovel on his 

 belly, and, standing well over the snake, 

 depict it from above ? He would do this, 

 and be delighted with the spirited result. 

 A shght alteration of the head would 

 complete the convention. 



In all seipents the skuU is modified to 

 admit of an enormous 

 distension of the jaws. 

 Perhaps the greatest 

 displacement occurs in 

 connection with the 

 mandibles. These, in 

 addition to being A\-idely 

 separable at the cliin 

 point, articulate loosely 

 with the mo\-able quad- 

 rate bones, which in turn 

 articulate with the elastic 

 squamosals {see page 

 554). Further stretching 

 occurs between the bones 

 which compose the roof 

 of the mouth, and the 

 final " gape " is limited 

 only by the elasticity of 

 the skuU muscles and 

 hgaments. The necessity 

 of some such speciah- 

 sation for dealing with 

 bulky articles of food is 

 evident, for it must be 

 remembered that a 

 snake's teeth perform no 

 masticating functions, 

 but are employed solely to steady the food, 

 while the snake works himself outside it. 

 Exigencies of space account for several 

 curious features in a snake's interior. 

 His \\'indpipe reaches well forward in his 

 mouth, and may e\'en loll out for breath 

 while the snake is dealing with a refractory 

 meal. His forked tongue lies normally 

 in a sheath beneath his windpipe. Only 

 one lung (the right) is fully de\-eloped. 

 The gullet is extremely long (some nine 

 inches in the average Adder) and admits 

 of at least the same distension as the 

 mouth. A small but strong constriction 

 separates it from the stomach, which is 

 comparatively short (two and a half to 

 three inches in the Adder), thick walled, 

 and only slightly distensible. 



One of Professor Leighton's interest- 



last he would ha\'e an inspiration, perhaps ing investigations on reptiles is concerned 

 in answer to an appeal to the god Cneph. with the digestion of snakes. 



