These four classes, be it observed, are only designated ‘roughly speaking.’ Nicholson describes a close gradation in the development of the poison glands also to correspond with those almost imperceptible stages. The poison gland is after all only a modified salivary gland. It lies behind the eye, whence the venom is conveyed by a duct to the base of the fang, down along it, and sometimes through it, and is emitted at what we may for the present call the point, into the wound made by it, something on the principle of an insect’s sting. As when inserting the sting the pressure forces the poison out of a gland at its base, so does the pressure of certain muscles act upon the poison gland when a snake opens its mouth to strike. In some of the most venomous, viz. the viperine families, the largely developed glands give that peculiar breadth to the head. There is a hideous, repulsive look about some of these, that seems to announce their deadly character, even to those who see one for the first time. The evil expression of the eye, with its linear pupil; the peculiar curve of the mouth, with its very wide gape downwards, and then up again, are unmistakeably treacherous, venomous, vicious. Like all other animal secretions, the poison is produced, expended, and renewed, but not always with equal rapidity; climate, season, and temperature, as well as the vigour of the reptile, influencing this secretion. The hotter the weather, the more active the serpent and all its functions. When the poison gland is full and the snake angry, you may see the venom exuding from the point of the fang, and by a forcible expiration the reptile can eject it. I have seen this in the little _Echis carinata_ and its congener the _Cerastes_. I am not certain whether the _Cerastes_ hisses or not, but under terror or excitement it moves itself about in ‘mystic coils’ as Echis does, producing a similar rustling noise with its scales; but both of them, if angry, will strike at you with a sound which may be compared with a sneeze or a spit, at the same time _gnashing_ their mobile fangs and letting you see that they have plenty of venom at your service. They may almost be said to ‘spit’ at you, though literally it is the mouth ‘watering with poison,’ combined with the natural impulse to strike, which produces this effect. We can, however, by this judge of the force with which the venom is expelled, which in a large viper must be considerable. Travellers have told us that a serpent ‘spouts poison into your eye.’ If an angry one strike, but miss its aim, the poison is then seen to fly from its mouth, sometimes to a distance of several feet. Whether a snake is so good a marksman as to take certain aim with this terrible projectile, or whether he possess sufficient intelligence to attempt it, we may doubt. Dr. Andrew Smith tells us that this belief prevails among the natives of South Africa.