GENERAL CHARACTERS. 177 



internal organs of snakes are long and narrow ; and it is remarkable that, as a 



rule, only one of the lungs is developed. 



Resembling; the other members of the order to which they 

 Teeth ■ 



belong in that their teeth are never implanted in distinct sockets or 



grooves, snakes exhibit some considerable degree of variation with regard to the 

 number and structure of their teeth. In the ordinary harmless forms there are 

 generally two rows of short, slender, and sharply-pointed teeth in the upper jaw, 

 the innermost of which are attached to the bones of the palate, while the lower 

 jaw carries only a single row of such weapons. One or two of the outer row of 

 upper teeth, either at the front or back of the series, may, however, be enlarged 

 beyond the rest, and grooved or tubular ; and it is probable that all snakes with 

 such a dental armature are more or less venomous. Some most deadly poisonous 

 serpents have, on the other hand, a type of dentition of their own ; and there is 

 no doubt that all snakes with teeth of this nature are extremely venomous. In 

 such snakes the forepart of the very short maxillary bone of each side of the 

 upper jaw is armed with an elongated tubular tooth, which ordinarily lies nearly 

 flat on the surface of the palate, but can be erected, by a peculiar mechanism of 

 the bones, when the jaws are opened. Although in this group the poison-fangs 

 are always tubular, in some of the other venomous serpents they are merely grooved 

 for the conveyance of the venom from the secreting gland ; but there is a transition 

 between the two types, as the closed tube is formed merely by the edges of the groove 

 being elevated until they unite in the middle line. In poisonous snakes, on each 

 side of the upper jaw, below and behind the eye, is situated the poison -gland — 

 merely an ultra-development of an ordinary salivary gland ; these glands in some 

 cases being so developed as to extend far back along the sides of the body. The 

 gland is overlain by a layer of muscles, for the purpose of forcing the secretion 

 into the tooth (the base of which is always open) when required ; this action 

 always taking place when the snake opens its mouth to bite. The poison then 

 flows along the channel or tube of the tooth, and is discharged at its extremity into 

 the wound. Considerable force is used in the emission of the poison, as, when a 

 snake is irritated, the fluid may be seen to spirt for some distance from its point of 

 discharge. In some of the less specialised poisonous snakes, the venom-tooth, 

 which has an open channel, is not greatly longer than the others, and is placed 

 nearly vertically when the mouth is closed. Although the poison-teeth are 

 commonly regarded as purely defensive weapons, their chief use is for the de- 

 struction of the prey of their owners, which is always killed before being swallowed. 

 The venom-tooth of the more specialised poisonous snakes is exceedingly likely to 

 be broken off during use ; but to take its place there are always several others 

 lying on the gum behind it in different stages of development. 



Harmless and Poi- Before the doctrine of parallelism in development received from 

 sonous Snakes, naturalists the attention it undoubtedly merits, snakes were generally 

 divided into harmless and poisonous groups ; but since we have become better 

 acquainted with that important factor in evolution, it has been recognised that 

 such a distinction is a purely artificial one, and has nothing to do with real affinity. 

 Certain groups of snakes, such as the members of the viper family, may, however, 

 be wholly poisonous ; while in other groups, such as the typical snakes, some 

 vol. v. — 12 



