Bag of the pharmr.—The masticated aliment is collected on the back of the tongue, which is then carried upwards, and backwards, to discharge it into the pharynx. This bag is covered by muscu lar fibres (forming the muscles called constrictores pharyngis) which contract, successively, in order to propel the food towartls the stomach. But as there are several organs communicating with the pharynx, the food might pass in a wrong direction, if the parts were not so con trived as to prevent such occurrences.
In the upper and anterior part af the pharrnx, the nostrils open by two large and ree apertures. Between these and the entrance from the mouth is found a fleshy and moveable curtain, called the soft palate, or velum pendulum palati. There is &small body-, of a pointed figure, projecting from the rtiddle of this organ, and known by the name of the uvula. This curtain and the uvula can be easily seen in the throat of a living person. It admits of being elevated so as to 'shut the opening of the nostrils ; and its ac. tion is exemplified in the act of vomiting: the food is forcibly thrown into the pha rynx, and would pass mostly into the nose, were it not prevented by the soft palate. From the uvula the membrane is continued on eit.her side, in an arched form, towards the root of the tongue, and it contains a glandular body, called the tonsil, which secretes a mucous fluid, to lubricate the parts, and facilitate the pas sage of the aliment. The larynx opens into the pharynx, just at the root of the tongue; over this part, which is termed the glottis, every morsel of the food must necessarily pass ; yet, so exquisitely ten der is the niembmne of tile wind pipe, that the contact of the smallest extrine ous body excites a convulsive paroxysm of coughing, that does not cease until the offenthngmatter be removed. Here then are two objects to be effected ; the func tion of respiration requires that the wind pipe should have a free communication with the external air, while the irritable nature of its membrane demands that no extmneous body- should find admission. These points are both attained by means of a strictly mechanical contrivance ; by a structure which produces the required effect, independently of the will of the animal, and merely in consequence of those motions which the organs perfortn in the office of deglutition. At the back of the tongue, and just in front of the glottis, is a cartilaginous valve, called the epiglottis. When the parts are at rest,
this valve stands perpendicular, and con sequently does notinterfere with the pas sage of the air into the wind-pipe. In the act of swallowing, the tongue is carried backwards, and the wind-pipe is drawn up : hence the epiglottis becomes me chanically applied over the opening,' and at this monicnt the food enters the pha rynx over it, and by its pressure closes the aperture still more completely. As soon as the food has passed, the tongue and vr-ind-pipe resume their former position, the elasticity of the cartilage restores it to the erect state, and the glottis is again free for the continuance of respiration. So completely does this simple mechan ism answer the proposed end, that, al though every morsel of food passes over the giottis, the accident of any portion going the wrong way, as it is termed, is comparatively'rare, and can only arise from our being^ imprudent enough to laugh or talk while we are swallowing. In either of these cases air must pass out of the trachea, and, by so doing, it lifts rip the epiglottis.
The pharynx opens below in the ceso pimp's, a muscular tube, which conv6-s t-he fbod into the stomach. The aliment, it' its farther progress, goes through dif ferent viscera contained in the abdomen ; and we shall therefore proceed with a de scription, of that cavity-.
The term abdomen includes a large portion of the body. It is bounded above by the cartilages of' the ribs, and by the diaphragm, which separates it from the chest, at the back part, bv the bodies of the lumbar vertebra ; in front and at the sides, by the abdominal muscles ; and be low, by the bones of thc pelvis.
It is every where lined by, a membrane called tile peritoneum. The surface of this is perfectly smooth and polished, and moistened by a serous exhalation produ ced by the minute arteries of the part. This membrane not only lines the cavity of the abdomen, but also covers all the viscera contained in that cavity, so that the exterior surface of each part consists of what anatomists call its peritoneal coat Hence the motions of these parts upon each other, and upon the surface of the cavity, are performed with perfect facility. The productions of the membrane, which gives these exterior investments to the viscera, seive also to confine them in their relative positions.