Usually, if nature undertakes experiments with poisons, it does not stop half-way, but creates something capable of inspiring real horror, as, for example, poisonous snakes whose bite may be fatal for man.
Now, from where does snake venom come? The venom is merely the snake's saliva secreted by somewhat modified salivary glands opening into a groove inside the tooth. The venom is only secreted when the snake bites, pressing against a little sac at the base of the tooth. During the bite all the venom is injected into the wound.
Some snakes proved very ingenious in developing their lethal weapon. Cobras (the rose and zebra type) and other African snakes perfected their technique of biting and are very good at spitting their venom. Their poisonous fangs differ somewhat from those of their fellow snakes. The groove along which the venom is ejected does not open out at the very tip of the tooth but some distance away from it (evidently to facilitate ejection), widening into a sort of funnel. For this reason, if the bite is not deep, the poison may not reach the wound, but disperses in fine drops over a wide area. As in a shot-gun the strike area is the larger, the greater the distance between the snake and the target.
Snakes are experts at spitting venom and have a range of up to four metres. This range is achieved by combining the pressure in the venom sac with the inertia of movement, achieved by jerking the head forward with the simultaneous ejection of the venom. If the venom gets into the eyes, the mucous membrane of the nose or the mouth of small mammals, they will die. Such a long-range weapon is more efficient than in other poisonous snakes.
Not only snakes have poisonous saliva. In the Pacific, near the Island of Fiji, New Guinea and Samoa there live gastropod (univalve) mollusks with beautiful, cone-like shells as long as 15 centimetres in length. However, one should not touch these cones. The crafty mollusk is sure to bite you with the sharp teeth of its radula. The poison of these creatures, especially that of the larger ones, is fatal to man.
In far from all cases are the teeth the best possible tools, and nature has not hesitated to substitute more perfect technical means for them. Many species of the prosobranchiate snails feed on mollusks which are rather large and enclosed in a hard shell. To make a hole in the shell with a radula would take weeks or even months, and the radula would wear out. Therefore, these snails use a specific saliva instead of teeth, which is a four-percent solution of sulphuric acid. Nor is this very strange, for if the glandular cells of man's stomach secrete hydrochloric acid, why should snails not make use of sulphuric acid?
The acid secreted by the snails is so strong that it hisses and effervesces when falling on marble. It dissolves the mollusk shells quite easily. When attacking their prey, the snails apply their saliva to the shell which loosens a small section of it. The preying snail then bores a hole with its radula, inserts its proboscis and is then able to enjoy eating the defenceless victim.
It is not always enough to crush food for it to pass easily into the gullet. This is why the 'preparatory shop' contains the large and small salivary glands for both the mechanical and chemical processing of food. Saliva performs many important functions, but the most important seems to be to wet each lump of food which otherwise would not pass into the oesophagus. Anyone who had a chance to observe the European pond tortoises was easily convinced of the importance of saliva. The pond tortoises have no salivary glands. They eat their prey in water, amply washing down each mouthful. But on land they are helpless since completely dry food sticks in their throats.
The saliva of most animals contains substances (enzymes) which are the first to act chemically on the food being taken in. Nature subsequently developed these properties, making saliva somewhat poisonous. This is necessary as numerous microorganisms, most of them harmful to the organism, may lodge in the moist membrane of the mouth and the remains of food stuck between the teeth.
Those creatures which have no teeth of their own have to resort to dentures. Food eaten by birds is crushed by little stones in their stomachs: the grains are taken into a thick-walled muscular stomach of considerable strength and crushed between the little stones as though between millstones.
Small stones are often to be found in the stomachs of birds. You may find them if you are preparing a chicken for the oven. But this phenomenon is still a puzzle in many respects. What makes birds swallow stones? They do not do it because they are hungry. How do birds know when the stones in their stomachs have worn out and need replacing? What makes them pick up only sufficiently hard stones? As yet, we cannot answer these questions.
Not only birds are fond of swallowing stones. Stones weighing from 350 to 500 grams are often found in the stomachs of whales, walruses and seals. From time to time they belch out these stones and this is why there is often a lot of stones from the sea bottom deposited in places where these pinnipeds spend a good deal of their time when on land. One might think that they decided to establish a geological museum on the sea-shore.
Scientists do not as yet know why sea mammals load their digestive tract with such unusual objects. The reason may be that stones help, as they do in birds' stomachs, to crush such hard parts of their food as mollusk shells and the chitinous shells of arthropods. Another reason may be that stones are used in the struggle with intestinal parasites, which are a particular nuisance to pinnipeds.
Animals are especially apt to swallow stones when they have gone for a long time without food. Hence, the suggestion has been made that swallowing stones prevents the stomach from atrophy (the shrinkage of an organ or tissue with resultant cessation of functioning). Thus, swallowing stones keeps the stomach busy when it otherwise has nothing to do.
However, it is quite probable that some sea mammals indulge in stone eating for reasons other than that of digestion. Some scientists believe that stones become indispensable when the mammals feed particularly well and get fat. As a result, their mean specific gravity drops and they rind it more and more difficult to submerge in water. Seafaring animals may also swallow stones as to take aboard some ballast and increase their weight. Calculations prove that the amount of ballast is quite large: some seals had as much as 11 kilograms of stones in their stomachs.
Teeth are also extremely important to predatory fish. Sharks have jaws whose inside surface is studded with teeth. These are arranged in regular rows with the tips pointing backwards, thus allowing the shark to hold its prey securely. Of course, the teeth at the very front have to work the hardest and they wear out the most rapidly. Sharks, too, would have a bad time if their front teeth were not replaced by new ones. The fact is that the front teeth are in motion throughout the shark's life. Bent over like attacking soldiers, row after row, they slowly but steadily move towards the edge of the jaw. The front rows of old worn-out teeth gradually 'crawl' out and, after having taken a glance at the outside world, fall out, only to be replaced by the next ones. Having worked their share and become well worn-out, these teeth, in their turn, release themselves and those behind move up to replace them. This process continues until the shark dies. Some extinct fossilized sharks had teeth which had not fallen out and, although they were quite old, they had the front part of their snout studded with teeth. This ability to constancy renew its teeth means that a shark is аble to fight right up to old age.
When the teeth are used solely to crush food, they may be located in some place other than the mouth. In some cases it may even prove advantageous to move them from the 'preparatory' shop to some adjacent department. Fish of the carp family have a toothless mouth, but you would be well advised not to put your finger into the throat of such a fish for it is there that they have their teeth and where the initial processing of food is carried out.
Some predatory fish and sea turtles have their teeth in their gullet. These are not properly teeth but very sharp and sometimes rather large spikes which are necessary to prevent the prey, which is still alive, from getting away. A spike-studded gullet is very much like the skin of a hedgehog or spiny ant-eater. All the spikes point towards the stomach and the food can thus only move in that direction. There is no way back from the stomach.
When asked to name the most important organs in the body, few people remember the teeth, but these, nonetheless, perform a very important function. The teeth frequently help to kill the prey, hold it and then break it up for food. This is why wild animals which have lost their teeth are doomed to death. Even man, who learned to make false teeth and is in no way limited as to his choice of food, is not indifferent to the loss of his natural teeth.
Teeth are equally important both to predatory and herbivorous animals. The well-known Indian hunter Jim Corbett describes several instances when the loss of but a single canine tooth made a tiger attack domestic animals and even humans, as he was no longer able to cope with the large hoofed animals on which he usually fed.
Rodents probably give their teeth more work than any other animal. Even the sharpest teeth cast from the hardest metal would be worn down by such work. The only solution is for the teeth to keep growing. In fact, the front teeth of rodents grow continuously and so quickly that if the animal were deprived of hard food and the teeth stopped wearing down, they would grow to an incredible size and incapacitate their owner. The incisors of rats grow three centimetres a month. If they did not wear them down, every tooth would reach 70 to 100 centimetres by old age.
The elephant's expectancy of life depends on the condition of its teeth. In a free state it feeds on vegetable matter some of which may be rather hard and has to be crushed by its powerful molars. An elephant has only two pairs of working teeth: one pair is in the upper jaw, the other in the lower. In addition, each jaw has five pairs of rudimentary teeth. As the teeth wear out, they fall out and new ones grow in their place until the sixth pair, which is the last, has worn out. The elephant's nutrition gradually deteriorates and this results in its death.