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4. Deglutition; 5. Action of the stomach; 6. Action of the small intestines; 7. Action of the large intestines; and, 8. Defecation.
Prehension, the first step in digestion, is an office which that admirable instrument the hand, the lips, cheeks, incisor teeth, and tongue perform for man. The lips are highly sensitive and muscular, and are encircled by arteries. The maxillary divisions of the trifacial nerve endow them with great sensibility, and if divided, as done by Sir G. Bell on an ass, the guiding sensation is lost, and the animal will not take up food although presented to him. Many muscles surround the mouth and are supplied by the facial nerve, the buccinator being also supplied by the motor part of the trifacial. The tongue, an organ hereafter described, acts in many animals as a prehensile organ, and in the infant aids suction by being drawn back into the mouth like a piston. When the lips are tightly applied round the nipple a vacuum is produced by the expansion of the mouth, and the milk is forced from the breast by the atmospheric pressure upon it. Coryza interferes seriously with suckling.
Mucous Membrane is continuous with skin at the lips, and its structural anatomy may be here briefly described. It forms two great tracts or continuous sheets, the gastro pulmonary and the genito-urinary, and in addition the mammary tract in the female. The intestinal subdivision has two orifices, the others are coecal. The membrane is termed simple when it has a plane surface, compound when folded inwards forming follicles, outwards forming villi or projections. Mucous membrane is everywhere composed of epithelium and basement layer.
Epithelium is of four kinds: 1. Flat, hexagonal or polyhedral cells, arranged in one or more layers. This is termed scaly or tesselated, and occurs in the mouth, pharynx, oesophagus, ureter, bladder urethra, and external female genitals. 2. Columnar, which is com
Tesselated ( Vagina).
posed of rod-like cells narrower at the end where they are attached. It covers the intestinal villi, air passages, &c. 8. Ciliated, which is but the columnar variety, with the addition of cilia to its free extremities, as in nasal, bronchial, Fallopian surfaces, &c., and these in every case wave towards the outlets. Cilia are found on the tesselated arachnoid epithelium, but never on the next kind of epithelium. 4. The glandular, consisting of oval or globular cells, which in various secreting organs have the .power of choosing the materials of the special secretion from the blood. The second constituent of the mucous covering is the basement layer, a homogeneous lamina, with, occasionally, a slightly fibrous appearance. It corresponds underneath to areolar tissue, containing the nutrient vessels. A basement membrane is said to be wanting under the hepatic cells.
Mastication is aided by the cheeks, which roll the mass from side to side, mould it, and prevent it clogging outside the teeth; and the tongue, which besides its gustatory powers, canby it s exquisite mobility remove the food from any part of the mouth, and keep it always under the operation of the teeth.
The Teeth are arranged in two semi-parabolic curves, the upper and lower dental arches, of which the upper is the greater and includes the lower. This is of use in
Glandular Epithelium, (Liner.)
dividing the food; and by mutual attrition, the teeth, instead of being blunted, as they would be if they came just together, are bevelled on the posterior surface in the upper ones, and the anterior in the lower. In adult man there are 82 teeth, 16 in each jaw and 8 on each
side, arranged from before backwards without any diastema, viz., 2 incisors for cutting, 1 canine for seizing, 2 premolar or bicuspid for tearing, and 8 molar for grinding the food, all contained within that "case of instruments," the mouth. These numbers are often expressed by Owen's dental formula:
The same number exists in many animals, but many variations occur between the extremes of 1 in narwhal and 190 in dolphin. The close unbroken apposition of the teeth occurs in man only of living creatures, but has been found in the fossil anoplotherium, nesodon, and dichodon.
A tooth is divided into a crown, which projects above the gum, a vascular tissue grasping it at a slight groove termed the neck, and a root, consisting of one or more fangs, set into the alveoli of the jaw-bones. Enamel coats the crown; cement or crusta petrosa, the fang; and all the rest is composed of dentine or ivory. The body is hollowed out into the pulp-cavity, compared to an Haversian canal, and which contains a mass of looped nerves
Section oj a Molar Tooth—c, cement; d, dentine; e, enamel,
and blood-vessels transmitted through the tube in each fang. These parts are shown in the foregoing section.
Dentine (d) consists of fine tubes running from the pulp-cavity to the enamel or cement, in such direction as to oppose pressure applied to the crown or sides of the tooth. To confer some elasticity, they take two or three wavings, and small curves even to the extent of 200 in a line. They often branch in binary order, and dilate into lacunae near the cement, thus forming the " granular layer" of Tomes. The dentine is marked by "counterlines," which evidence stratification. The dental tubes are of very fine calibre, their wall being thick in proportion, admitting perhaps the thinnest part of the blood. Madder does not stain the tooth. Dentine contains about 72 per cent. of earthy salts. When teeth first appear, the crown is covered with a thin layer of cement, which soon wears away and exposes the enamel coated with a fine pellicle.
Enamel («), having but 2 per cent. of organic matter, is the hardest substance in the body, and will strike fire with steel. It is arranged in pentagonal or hexagonal rods ti^u in diameter, extending from the dentine to the surface of the crown, where they present an exquisitely smooth surface. Towards the dentine the rods are of unequal length, corresponding to that of dental tubes. High microscopic power shows transverse markings on the rods, connected probably with their development or nutrition, which may be also accomplished by the very slight spaces they leave between each other. The sensation produced by acid, when the teeth are said to be "on edge," is hard to account for, as no nerves probably exist in dental tubes, or between enamel-rods, but these may convey an impression to the nervous pulp.
The Cement (c) is a layer of bone crusting the fangs, and thus setting the tooth in its socket. Albinus' museum contains an upper molar with the fangs growing down, the crown in antrum. Up to 1839, dental tissue was described as a " dead part or product exhaled from the surface of a formative bulb," but their exquisite