All animals, man included, have doubtless in their saliva a deadly poison, though in the latter it is extremely diluted, and essential only to the digestion of food. In poisonous snakes, however, it is stored up in sacs, modifications of the salivary glands, and placed in each side of the upper jaw. From the poison-gland under the eye forward to the edge of the jaw, a delicate canal, which opens into the fang above the tube of the tooth, extends. Alongside of the latter may be seen rudimentary fangs, all ready to grow out should the large one be lost. To use the poison, the snake has merely to strike its prey, when the muscles of the jaw, which are admirably fitted for the part they have to play in the tragedy, press upon the glands, squeeze the poison through the little canal down through the hollow fang, and the work is accomplished. In their actions, snakes are most graceful. The gliding motion, so characteristic, is effected by the movements of the large central scales, that are successively pushed forward, the hinder edges resting on the ground and constituting a support. These scales, or pushers, are fastened to the ribs by muscles, and by holding a snake by the hand the swelling movement can readily be felt. Snakes vary much in color. They are generally adapted to their surroundings. Green Snakes are found in green grass and vegetation, while grey snakes affect rocky districts, where they are alike protected. Their skin is shed in one piece at various seasons of the year, being forced off by the snake forming a ring with its tail and squeezing the rest of the body through it, or by wriggling through entangled bushes. Poisonous snakes may be always recognized by their broad, flattened heads, generally short and thick bodies, and the almost invariable possession of a vertical keel along the centre of each scale. Long bodies, small heads devoid of distinct necks, and scales not keeled, characterize non-poisonous species. Probably the best-known of our common kinds of poisonous snakes are the rattlesnakes. They belong to the dangerous family Crotalidæ, to which the copperheads and moccasins also belong, and are distinguished by the large, ugly head, absence of teeth in the upper jaw excepting the fangs, and the pit in the head. [Illustration: NORTHERN RATTLESNAKE. Prepared to Attack a Song Sparrow.] _Crotalus horridus_, our Northern Rattlesnake, has doubtless the widest geographical distribution, being found in nearly every State in the Union, from the Gulf of Mexico to Northern New England, and thence west to the Rocky Mountains. It has a most forbidding appearance, and when once seen with its enormous head, triangular in shape, and large brilliant eyes, with fiery irides, it can never be mistaken. Between the eye and the nostril is a deep pit, a character that is peculiar to the family.