552 



THE NATURE BOOK 



This appears to be effected by instal- 

 ments, and it is likely, but not certain, 

 that the speed of it can be controlled. 



Having secured a tooth grip of his victim, 

 the snake proceeds by slow degrees to push 

 himself outside it. By the time he has 

 accomplished this feat, the skin surround- 

 ing his gullet is stretched to such an 

 extent that every scale on it is parted 

 from its neighbour ; and, should the 

 snake be startled at tliis juncture, he 

 will frequently disgorge his meal. Instance 

 after instance has been recorded of frogs 

 so disgorged, being apparently none the 

 worse for the temporary inconvenience 

 of being swallowed. 



No digestion can take place in the 

 guUet. The nutrition of the snake is, 

 therefore, of necessity deferred until the 

 portion of his victim first swallowed 

 (in the case of mice this is usuaUy the 

 head) has passed from the gullet to 

 the stomach. Here, as might be sup- 

 posed, the main process of digestion is 

 accomplished ; but further digestion occurs 

 in the intestine, and the percentage of 

 actual waste per meal is extremely small. 

 The interest of Professor Leighton's inves- 

 tigation lies in the suggested possibility 

 that snakes can let digestion wait on 

 appetite by retaining undigested food 

 in the guUet for as long as they feel in- 

 clined. This would account not only 

 for the lengthy intervals between snakes' 

 meals, but also for their bulk. Of the 

 latter. Professor Leighton gives an inter- 

 esting instance. He kihed an Adder 

 which had swaUowed two young water 

 rats. The head of number one had 

 reached the stomach and had been par- 

 tially digested ; number two, which was in 

 the fore part of the guUet, had no signs 

 of digestion on it. It is difficult to avoid 

 the conclusion that this Adder had 

 intended to provision himself for some 

 weeks in advance. 



An Adder's food consists of small rats, 

 mice, lizards, and fledgling birds. It is 

 not likely that he habituaUy expends 

 poison on victims of this description. 



The opening of his jaws does not 

 necessarily invoh'e pressure (m the poison 

 glands, and he can lower or raise one or 

 both fangs at will. It is probable, there- 

 fore, that he reserves his poison for use 

 in self-defence, or for dealing with some 



particularly large or refractory victim. 

 The mechanism by which a snake's poison 

 apparatus is controlled is exquisitely 

 balanced. It differs in the various 

 families of snakes ; but in all the wounds 

 inflicted is a bite, and the venom comes 

 through or along the tooth wliich inflicts 

 the wound. 



The venom of Adders is secreted and 

 stored in two pear-shaped glands (corre- 

 sponding to the parotids in a human being) 

 the rounded ends of which lie towards 

 the back of his head. Usually, but not 

 in\'ariably, the presence of such glands 

 causes the head of a venomous snake to 

 appear flattened and broadened behind. 

 Ducts lead from these glands to the fangs. 

 In the case of some venomous snakes 

 the fangs are grooved in front, and the 

 poison travels down the grooves as the 

 snake strikes. This is probably a 

 primitive condition. In the Adder the 

 lips of the external grooves have coalesced, 

 and each poison fang consists of a taper- 

 ing recurved tube the upper orifice of 

 which communicates with the duct from 

 the poison gland, and the lower orifice 

 of which is, at its commencement, a little 

 short of the actual point. 



This " perfectly devilish contrivance," 

 as Mr. Gadow puts it, ensures that the 

 injection of venom shall be deep down 

 in the wound, and that it shall have a 

 clear run. The hypodermic syringe, in 

 which, to avoid clogging, the point is 

 bevelled so that the orifice shall commence 

 a httle short of the extremity, is a practical 

 apphcation of the same principle. In the 

 case of the sjTinge a plunger expels the 

 fluid ; in the case of the snake equivalent 

 force is supplied by muscular pressure on 

 the poison glands. 



The fore portion of an Adder's jaw 

 (maxilla) in which the fangs are implanted 

 admits of such displacement in an up- 

 ward direction, that the fangs can be 

 swung from a position in which their 

 points are directed towards the reptile's 

 palate, to one in which, though their 

 points still curve backward, their main 

 axes are nearly vertical. This ensures a 

 dragging wound and a freer injection of 

 the poison. The displacement {see 

 page 554) is effected by linkage between 

 the quadrate, the pterygoid, and the 

 ecto-pterygoid bones, fixed pivots being 



