Teeth , Tooth structure, Growth of teeth, Herbivores & Dentition in man

 


TEETH are produced by the skin where it covers the jaws. Their roots become enclosed in the developing jaw bone, and their crowns break through the skin into the mouth cavity. They are thought to have arisen in the course of evolution from the scales of fish such as the dogfish. Where the skin bearing the scales stretched over the jaws, the scales became enlarged, pointed and specialized for holding prey or breaking up food.


Tooth structure.

Enamel is the hardest substance made by animals. It is deposited on the outside of the crown of the tooth by cells in the gum before the tooth reaches the surface. The enamel is a non-living substance containing calcium salts; it forms an efficient, hard, biting surface.

Dentine is more like bone in its structure. It is hard but not so brittle as enamel and running through it are strands of cytoplasm from cells in the pulp. These cells are able to add more dentine to the inside of the tooth.

Pulp. In the centre of the tooth is soft connective tissue called pulp. From this the living strands of cytoplasm in the dentine derive their food and oxygen. The pulp contains sensory nerve-endings and blood capillaries. Oxygen and food brought by the blood enable the tooth to live and grow. The nerve-endings are particularly sensitive to heat and cold but produce only the sensation of pain.

The root is not set rigidly in the jaw bone, but is held by tough fibres so that it moves slightly in its socket as a result of Chewing and biting movements.

Cement is a thin layer of bone-like material covering the dentine at the root of the tooth. The fibres which hold the tooth in the jaw are embedded in the cement at one end and in the jaw bone at the other.

Specialization of teeth: carnivores

Where all the teeth serve the same purpose, such as merely holding the prey to prevent its escaping, they are all very Similar in structure. In most carnivorous fish the teeth are Simply sharp pegs projecting backwards. In such animals the prey is often swallowed whole without chewing.

Where the food is captured, broken up and chewed, the teeth in different regions of the mouth may become specialized to a particular function. For example, a dog's incisors in the front of its mouth meet and can grip and strip off small pieces of flesh close to the bone. The long pointed canines are near the front and penetrate the prey, preventing its escape and often killing it. The massive carnassial teeth have sharp cutting edges. They pass each other and act like shears or snips, slicing off flesh and cracking bones. The molars have more flattened surfaces and meet each other, so crushing the bones and flesh to smaller particles before swallowing. The molars and premolars develop near the back of the jaw where the mechanical advantage is greater.

The dog's dentition is characteristic of carnivorous mammals, i.e. those which feed on other animals. Animals which feed exclusively on vegetation are called herbivores. The differences in shape, size and distribution of the teeth in these types of animal show adaptations to the differences in diet. Their ali- mentary canals also differ, herbivores having very long intestines and well developed caecum and appendix, features probably associated with the slowness of cellulose digestion. Flesh is digested relatively more rapidly and the intestine of carnivores is short with small caecum and appendix. 


Growth of teeth

The teeth of herbivores have wide openings at the roots and grow throughout the life of the animal. In carnivorous and omnivorous mammals, including man, the teeth cease to grow beyond a certain size. This may be due to the gradual closing of the hole in the root, which constricts the blood vessels and reduces the blood supply, so preventing further growth.

Most mammals have two sets of teeth during their lives. The first set, or milk teeth, fall out as a result of their roots being dissolved away in the jaw, and they are replaced by the permanent teeth. A human two-year-old baby has 20 milk teeth which are replaced after the age of 5 years by 32 permanent teeth. These may not all appear until the age of 17 years or more.

Herbivores

The permanent teeth of herbivorous mammals continue to grow throughout the animal's life, being constantly worn down.

The lower jaw moves sideways or backwards and forwards grinding the teeth across each other and wearing away first the cement, then the enamel, and finally the dentine. The molar and premolar teeth of the upper and lower jaw ground together in this way come to fit each other exactly. The enamel, being the hardest of the three layers, is siower in wearing down and stands up in ridges with very sharp edges.

The grass and vegetation is ground and crushed between these exactly fitting edges, a high proportion of the cellulose cell walls are broken down so providing a greatly increased surface. Vertebrates, having no enzyme for digesting cellulose, depend on bacterial action for its digestion. An increased surface accelerates this process and the easily digestible protoplasm and cell sap are released from the crushed cells.

Herbivores canine teeth, if present at all, are usually indistinguishable from incisors and the toothless gap between the incisors and premolars allows the tongue to manipulate the food. The premolars and molars are almost identical in shape and size, as might be expected from the fact that they have the same functions. Many herbivores have no incisors or canines in the top jaw. The grass or other vegetation is gripped between the bottom incisors and the gum of the upper jaw.

Jaw articulation. In the herbivores, the joints between the lower jaw lower jaw and skull are fairly loose, allowing the sideways or back and forth movement of the lower jaw. In carnivores the jaw muscles, particularly the temporal. are very powerful and the hinge joint allows only up and down movement of the lower jaw, an adaptation which probably prevents dislocation of the jaw by (a) the strong chewing muscles and (b) the struggles of the prey.

Dentition in man

In man, the top and bottom incisors can pass each other when the jaws close and so cut off manageable portions of food.

The molar and premolar surfaces meet and serve to crush the food. The canines are little different in shape or size from the incisors. Human dentition does not show the same degree of specialization as seen in the carnivorous and herbivorous mammals.


Teeth , Tooth structure, Growth of teeth, Herbivores & Dentition in man Teeth , Tooth structure, Growth of teeth, Herbivores & Dentition in man Reviewed by Admin on October 11, 2022 Rating: 5

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