OPHIDIA. 165 



The skin does not form eyelids ; the part of the epidermis which covers the eye is trans- 

 parent, convex, and has the shape of a watch-glass, behind which the eye moves. The 

 epidermis is cast off in a single piece several times every year. 



Generally the snakes are provided with numerous teeth, which are elongate, conical, thin 

 and pointed like a needle, and more or less bent backwards. In the first suborder, that of 

 non-venomous snakes, the teeth are either entirely smooth, or only the last of the maxillary 

 series is provided with a faint longitudinal groove, which is not intended to convey a 

 poisonous saliva into a wound, as the saliva of these snakes has never been proved to be 

 poisonous : the groove appears to increase the strength of the tooth. Many of the non- 

 venomous snakes have long teeth in front of the jaws or of the palate, but they are never 

 grooved or perforated, and only serve to afford a firmer hold on the living and struggling prey. 



The pmsonous snakes are armed with a long canaliculated tooth in front of the upper jaw ; 

 the channel terminates in a small slit at the extremity of the tooth, and is in connexion 

 with a duct which carries the poisonous fluid from a large gland to the tooth. This venom- 

 gland corresponds with the parotid salivary gland of the Mammals, and is situated on the 

 side of the head, above the angle of the mouth ; it is invested by a dense fibrous sheath, 

 which is covered by a layer of muscular fibres. At the moment the snake opens its mouth 

 to bite, these muscles compress the gland, and force its contents through the excretory duct 

 into the channel of the venom-tooth, whence it is injected into the wound. We may judge 

 of the force of this pressure when we read the accounts of travellers Avho have seen irritated 

 animals spouting the poison from the aperture of the tooth to a short distance. The venom- 

 apparatus serves these creatures not only for defence, but also and chiefly for the purpose 

 of overpowering their prey, which is always killed before they commence to swallow it. 



The structure of the venom-tooth is not the same in all poisonous snakes : in some it is 

 fixed to the maxillary bone, which is as long, or nearly as long, as in the non-venomous 

 snakes, and generally bears one or more ordinary teeth on its hinder portion. The venom- 

 tooth is fixed, more or less erect, not very long, and its channel is generally visible as 

 an external groove. The poisonous snakes with such a dentition have externally a more 

 or less striking resemblance to the non-venomous serpents ; and on this account they are 

 designated as venomous colubnne snakes, forming the second suborder of snakes (Cobras, 

 Bungarums, Sea-snakes, &c.). 



In the other venomous snakes, the third suborder, the maxillary bone is extremely short, 

 and does not bear any teeth except an exceedingly long fang, with a perfectly closed, 

 externally invisible channel in its interior. Although this tooth also is fixed to the bone, 

 the bone itself is very mobile, so that the tooth, which is laid backwards when at rest, can 

 be erected the moment the animal prepares to strike. This tooth, like all the other teeth, 

 is not only occasionally lost, but appears to be shed at regular intervals. From two to four 

 other venom-teeth in different stages of development, destined to replace the one in action, 

 exist between the folds of the gum, and are not anchylosed to the bone. 



Most of the snakes feed on living animals, — only a few on eggs : some of them first kill 

 their prey by poisoning it, as we have mentioned, or by smothering it between the coils of 

 their body ; others swallow the struggling victim ali^ e. As they do not possess organs for 

 tearing the prey to pieces, nor a dentition fit for mastication, the prey is swallowed entire ; 

 and in consequence of the great width of the mouth and of the extraordinary extensibility of 



