82 



Curved pincers. The grinders are principally employed in grinding sub- 

 stances previously divided by the laniary teeth, which tear them, or by in- 

 cisors, which, in meeting as the blades of scissars, fairly cut them through: 

 the latter, of which each jaw contains four, acting only on bodies which 

 present but a slight resistance, are placed at the extremity of the maxilary 

 lever. The grinders are brought nearer to the fulcrum, and it is on them 

 that the great stress of mastication rests. If we wish to crush a very hard 

 substance v, e instinctively place it between the last large grinders, and by 

 thus shortening considerably the lever, between the resistance and the ful- 

 crum, we improve on the lever of the third kind, which though most em- 

 ployed in the animal economy, acts the most unfavourably. Xhe laniary 

 teeth have very long fangs; which lying deeply buried in the alveolar pro- 

 cesses, give them a degree of firmness to enable them to act powerfully, 

 without danger of being loosened from their situation, 



The enamel which covers the teeth, preserves the substance of the 

 bone exposed to the contact of the air, from the injurious effects, which 

 would not fail to result from direct exposure, and as enamel is much hard- 

 er than hone, it enables the teeth to break the hardest bodies without in- 

 jury. The concentrated acids soften this substance, and occasion a pain- 

 ful affection of the teeth. The sensibility possessed by these bones, is 

 seated in the mucous membrane which lines their inward cavity, through 

 which are distributed the vessels and nerves, which enter by openings at 

 their roots. This membrane is the seat of a great number of diseases, 

 to which the teeth are subject. The enamel, incessantly worn by repeat- 

 ed friction, grows and repairs its waste. The alveolar processes which 

 receive the fangs of the teeth, firmly embrace them, and all of them be- 

 ing exactly conical in form, every point of these small cavities, and not 

 merely their lower part at which the nerves and vessels enter, supports 

 the pressure which is applied to these bones. When from accidental 

 causes, or in the progress of age, the teeth are gone, their alveoli con- 

 tract*, then disappear; the gums, a reddish and dense membranous sub- 

 stance, which connects the teeth to the sockets, harden and become cal- 

 lous over their thinned edges. Old men who have lost all their teeth, 

 masticate but imperfectly, and' this circumstance is one of the causes of 

 their slow digestion, as the gastric juice acts with difficulty on food, 

 whose particles are not sufficiently divided. 



X. Salivary Solution. The above mechanical trituration is not the only 

 change which the food undergoes in the mouth. Subjected to the action 

 of the organs of mastication, which overcome the force of cohesion of 

 its molecules, it is at the same time imbued with the saliva. This fluid 

 secreted by the glands placed in the vicinity of the mouth, is poured, in 

 considerable quantity, into that cavity during mastication. 



The saliva is a transparent and viscous fluid, formed of about four parts 

 of water and one of albumen, in which are dissolved, phosphates of soda, 

 of lime, and of ammonia, as well as a small quantity of muriate of soda; 



which covers them externally. It has been said, with justicef, that Nature, in sheath- 

 ing- the tooth with this covering, hatf imitated the process of tempering 1 , by means of 

 which, we harden the edge of steel or iron tools. Author's Note. 



* When the teeth of old persons are removed, or fall out, the alveoli are gradually, 

 and entirely absorbed ; this is what is meaiit by their contraction. Godman. 



f We should say very ricTiculouslv, as no analogy exists between the processes." 



