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Chapter: Modern Medical Toxicology: Organic Poisons (Toxins): Venomous Bites and Stings

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Snakes - Organic Poisons (Toxins)

Snakes (also referred to as serpents) are limbless creatures with elongated bodies covered with scales.

SNAKES

Snakes (also referred to as serpents) are limbless creatures with elongated bodies covered with scales. The body is divided into head, trunk, and tail. The head may be oval, triangular, or the same width as the trunk, ending as a blunt snout. The head bears two eyes, two nostrils, and a mouth. External ears are absent since snakes do not posses auditory apparatus. However, though a snake cannot perceive sounds, it senses vibrations on the ground, which enables it to hunt prey and keep clear of predators. The eyes of a snake lack eyelids, each being covered by a transparent scale. Pupils may be round or elliptical.

The mouth of a snake is extremely distensible, enabling it to swallow large animals whole, without mastication. Snakes are essentially carnivorous and feed mostly on mice, rats, lizards, or frogs. Water snakes feed on fish. Some are cannibals and feed on other snakes, e.g. krait, king cobra, etc. Snakes can survive for long periods of time without food, even up to several months or years. Most snakes have four rows of teeth in the upper jaw, and two rows of teeth in the lower jaw. Two of the upper rows are situated along the margins of the jaw, while the remaining two are located on the palate near the middle of the roof of the mouth (palate teeth) . The lower rows are located along the margins of the lower jaw. Venomous snakes have modified teeth called fangs in addition to ordinary teeth. Fangs are usually two in number, invariably located one on each side of the upper jaw. They may be grooved or canalised, and are connected to the venom glands which are a pair of modified parotid salivary glands located one on each side, just below and behind the eye. All teeth are generally directed backwards which help in propelling a swallowed prey inwards, thereby minimising the possibility of it being disgorged.

The tongue of a snake is forked and can be flicked in and out of the mouth rapidly. It is not adapted for licking or sucking, but is actually a device to pick up scent particles from the exterior and transfer them to the Jacobson’s organ in the roof of the mouth. Thus the snake is able to track its prey, locate suitable mates, and smell out predators such as mongoose or humans.

The nostrils enable a snake to inhale air into its lungs. Hissing is accomplished by the forcible expulsion of air through the nostrils, and is an act of aggression or defence. Russell’s viper and puff adder are noted for their very loud hiss. In some (Crotalid) snakes, there is a pit between the eye and nostrils, which is a heat sensitive organ that helps in detecting warm-blooded prey.

Venomous snakes are found all over the world, except New Zealand, and most parts of the Arctic and Antarctic regions, as well as Ireland, Iceland, Chile, Hawaii, parts of Mediterranean and Caribbean regions, and some of the Pacific islands. In Britain, there is only one indigenous snake (Vipera berus), which seldom causes serious envenomation.








Snakes are encountered in all kinds of geographical loca-tions, such as on land, in marshes, and in water (fresh water and the sea). Most snakes can climb trees, and a few can burrow into the ground. Land snakes can swim in water, but sea snakes cannot survive on land. Snakes are cold-blooded creatures, and their body temperature varies depending on the environmental conditions. They usually hibernate in the winter in secluded places, and go without food and water for several months. Stored body fat helps in sustenance during hibernation. However, tropical snakes may not hibernate at all, and may be seen in the open all year round.

Snakes regularly moult, i.e. they shed their skin periodically (usually every 2 months). The skin which is cast off, is turned inside out, in the manner of a glove being removed from the hand. Moulting enables a snake to become more alert and active.

The trunk (i.e. the main body) of a snake may be stout or slender, and may be variously coloured. Often there are vivid patterns or designs. The portion of the body distal to the vent is called the tail, which may end abruptly to a blunt point, or may taper gradually to a fine point. The vent is an opening situated in the posterior part of the undersurface of the body, which serves as a common orifice for the alimentary and genito-urinary tracts. The tail of a sea snake is flattened and paddle shaped, to enable it to swim.

Sexes are distinct in snakes. Each species breeds true, and hybrids are virtually unknown. Most snakes lay eggs (oviparous) while a few species bring forth their young alive (viviparous). Snakes usually survive for many years, a few species living up to 20 years or more.

 

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