',ORGANS CONCERNED IN THE REDUCTION AND ASSIMILATION OF THE FOOD.
Organs of mastication and deglutition. The two jaws, with,their teeth, and the tongue, are the principal agents in the business of mastication.
The articulation of the condyle of the lower jaw with the glenoid cavity of, the temporal bone admits of part being moved in vattiods directions. Its depression and elevation cause the open ing and shutting of the mouth. It can be brought forwards, and carried backwards ; and admits also of being moved to one side or the other. It is by a combination of these various motions that the food is masticated, or reduced into a soft and pulpy form. The different teeth which are placed in various parts of the cavity of the mouth are adapted, by their form and situation, for various parts of the pro cess of mastication. The anterior ones, which have a thin edtting edge, and in which the superior overlap the inferior, act like the blades of a pair of scissors. These cut the food into smaller morsels; and serve us also in biting off a portion from any mass of food which we may be eating. The back teeth have brOad bases, furnished with obtuse prominences ; and they shut perpendicularly on each other. These are therefore well adapted for the grinding and trituration of the food. As their office requires a greater muscular force, they are placed in the back of the mouth, near to the centre of motion, and where, consequently, the action of the muscles is felt with the greatest effect. The cutting teeth are placed in front, at a greater distance from the attachment of the muscles, because, their office does not require so great a muscular exer tion.
The tongue is of considerable utility in contributing to mastication, as it serves to move the food about in the cavity of the mouth, and to subject it again to the ac tion of the grinding teeth, when it has escaped from between their surfaces. The muscles of this organ, which we have enumerated in the myological division of the article, give It a power of motion in every direction.
But the simple act of mastication would wily reduce the food into a powder, or at all eventsinto a dry mass, that could not be swallowed withoutgreatdifficulty. To obviate this inconvenience, it is plentiful ly moistened with a-watery fluid called saliva, and is thereby converted into itsoft paste, which can be conveyed into the stomach with perfect facility. The source
of this fluid ison several glandularbodies, situated near the mouth, and sending ex cretory duets, which conve,y the secret ed thud into that cavity. As the jaws move, the muscles compress th e se, glands, and squeeze the secreted fluid into the mouth. The tongue is constantly em ployed nging again ,under the action of the. teeth those porfions of the food which escape from between them ; and the closure of the lips prevents it from falling out of,the mouth.
The true salivary glands are three in number, on each side of the bead. The largest is placed in the space left between the ear and the lower-jaw-bOne; and is called, fromits situation, the parotid. Its duct pierces the middle of the cheek. The two others- are placed under the tongue, and are.called the submaxillary and sublingual. Their duets join to open by a common orifice, at the side of the membrane called the frenurn of the tongue, which ties the under-surface of that °Trail to the inside of the lower jaw. Besides these large salivary glands, there are other small granular bodies, which pour a mucous fluid into the mouth ; these are named, according totheir situa tion, glandula labiales, buccales, The cavity of the mouth in which the process of mastication goes on is not a very extensive one. There is a small space left between the cheeks and the teeth ex-ternally; but within the teeth the tongue occupies nearly the whole room. The upper boundary is formed by the pa.. late or roof of the mouth, and the lower by the surface of the tongue. The mouth opens behind, by a tolemble free commu nication, into a membranous bag, called the pharynx. The surface of the mouth is every where covered by. a soft and sinboth membrane. This is of course kept constantly in a moist state, as t.he glands above enumerated continually- pour more or less of, their secretion into the cavity. The membrane of the mouth is continuous uith the external surface of the body ; but the skin assumes a more delicate organization, as must be appa rent to every- body, from the change of colour at the lips.