Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/727

Rh NUTRITION 669 by the simple or branched tubular gastric glands. In the duo denum we find them as simple tubular deep sockets, the glands or crypts of Licberkiilni, and also as the compound acinous glands of Brunner, which dip below the level of the mucosa and lie in the submucosa. In the jejunum, ileum, and large intestine they appear again as the crypts or follicles of Lieberkiilm. The membranous tube is not everywhere exactly concentric and conterminous with the muscular tube. In the mouth and pharynx it is almost entirely so, but at the gullet the muscular tube so tightly encloses the membranous that the latter is forced into longitudinal plications to find room for itself. In the stomach the membranous layer is raised into ridges or rugse, which intersect and give the surface a honeycomb-like aspect. In the upper part of the small intestine the membranous tube is raised into deep annular or, more correctly, crescentic folds, running across the direction of the gut like incomplete diaphragms, or a series of membranous ledges; these are the valvulae conniventcs. In the colon the two tubes are so disposed as to form a regular series of saccules or pouches, adding largely to the capacity of the gut. All these foldings greatly increase the superficies of the membranous tube. A further enlargement is effected in the small intestine in an ex ceedingly interesting fashion; the surface of the mucosa is thickly studded with innumerable fine short projections resembling the pile of velvet. These are invested by surface epithelium, and amongst them, at their feet, open the before-mentioned crypts of Lieberkiihn. They are the so-called villi. Each contains a lymph atic vessel, blood-vessels, and involuntary muscular fibres, all sup ported by adenoid connective tissue like that of the mucosa below; the lymphatic is in the axis of the villus, the muscles form the next layer, and the blood-vessels lie immediately beneath the epithelium. When the muscular layer of the villus contracts it must of necessity compress the lymph-vessel, whilst causing no impediment to the flow of blood. We have described the mucosa of the stomach and intestines as containing a framework of adenoid reticular tissue like the tissue of lymphatic follicles. It is, indeed, identical with this, a net work of branched cells with oval nuclei, the meshes of which are crowded with lymph-corpuscles with round nuclei. At certain points in the intestines the adenoid tissue of the mucosa presents local nodular enlargements; the mucosa at these points becomes so much thicker that it swells up at the free surface beneath the epithelium into rounded eminences about as large as millet-seeds or the heads of small pins; and at the under surface of the mucosa it dips into the submucous tissue in a similar manner. At the base of this nodule of. adenoid tissue in the submucosa there is usually a network of wide, thin-walled, lymphatic vessels. Many of these rounded masses are scattered irregularly over the small and large intestines as the solitary follicles or glands, but at the lower end of the ileum they form little colonies, often covering an area an inch or more in length, situated at that part of the intestine which is remote from the attachment of the mesentery. They then con stitute the so-called Peyers patches. Nodular adenoid masses are, however, not limited to the adenoid mucosa of the intestines. They are occasionally, though rarely, found in the stomach; they exist beneath the mucous membrane of the tongue, and a colony of them forms the mass of the tonsil on each side, the almond-shaped body situated between the posterior and anterior pillars of the fauces. The intrinsic nerves of the alimentary tube consist of two sys tems of nervous networks with ganglion-cells lying at the nodes; one is found in the submucous layer (plexus of Mcissncr], the other lies between the longitudinal and circular muscles (plexus of Aucr- bach, or plexus mycntcricus). The whole of the intestines, and the stomach as well, are sus tained in the abdominal cavity by sheets of delicate membrane, formed by folds of peritoneum, and called, in the case of the intes tinal portion of the tube, the mesentery. Between the layers of the mesentery run the vessels and nerves for the supply of the bowel. In addition to blood-vessels there are numerous thin-walled lymph atic vessels called lacteals, which are fed by the rich network of lymphatic vessels of the mucosa and submucosa, and which run in the mesentery to the back of the abdominal cavity. Here they are collected into a large lymphatic reservoir, the receptaculum chyli, from which a duct, the thoracic duct, proceeds along the side of the vertebral column to open into the venous system at the j unction of the subclavian and jugular veins on the left side of the neck. The lacteal and lymphatic vessels, whose course has been briefly sketched, are interrupted at many points by the presence of lymph atic glands. These may be simply regarded as labyrinthine sys tems of vessels into which the simple afferent lymphatic or lacteal vessels open, and each of which is surrounded and penetrated by adenoid connective tissue, like that of the intestinal mucosa. After a meal the lacteal vessels are filled with a milky fluid, the chyle. They were discovered by Aselli in the year 1626. It must not be thought that the glands of the mucous membrane of the alimentary canal are alone engaged in the preparation of the solvent digestive fluids. Other organs there are lying away from the alimentary canal which pour their secretions into it by ducts at various points. In the neighbourhood of the mouth there are three pairs of salivary glands. The parotid glands lie outside the cheek over the lower jaw, just in front of the meatus of the ear Ihe ducts of this pair course along the cheeks and pierce them opposite the second upper molar tooth on each side; these are the two ducts of Stenson. A second pair of glands, the submaxillary, lies be neath the ramus of the lower jaw and beneath the floor of the mouth; their ducts run forward to open beneath the tongue; these are the ducts of Whar- ton. The third pair, the sublinyual, lies on the floor of the mouth, beneath the mucous membrane anterior to the openings of the ducts of Wharton. The ducts of these glands are numer ous and open by many apertures on the floor of the mouth, some opening into the ducts of Wharton. These constitute the ducts of Rivinus. About three or four inches below the pylorus the ducts of two large glands open into the small intestine by a common orifice; these are the ducts of Wirsung, or pancreatic ducts, from the pancreas on the left, and the common bile-duct from the liver 011 the right. Movements of the Alimentary Canal. When food is introduced into the mouth it is submitted FIG. 2. Nutritive System of Do to the operation of mastication. ^^S^^Kff^- The incisors or cutting teeth ln&amp;gt;ach : F ipfcen; G, re cep tacu- ,..,..., . r i i lum cliyli; H, pancreas; I, duo- dlVlde it into portions Of suitable denum; J, entrance of biliary size, which are thrust under the flttftftiSlftSS! broad faces of the molar or grind- O, rectum; P, kidneys, with the ing teeth by the alternate move- mentS of tongue and cheeks. c, ,. c c c ^ hotter portions OI food are crushed between tomme and hard .ea DCTO een tongue ana nara palate, especially SUch as are , i f ,1 i , taken for their Sapid properties to please the palate and increase ,1 L a f V ,, ,. ,. tlie now of Saliva. Mastication prmif nf mnvPTYiPnta r&amp;gt;f tViP consists 01 movements or the lower jaw Upon the Upper, viz., (1) vertical movements brought about by the temporal and the internal pterygoid and masseter muscles on the one hand, and by the digastric, mylo-hyoid, and other depressor muscles attached to the lower jaw on the other; (2) horizontal movements, for wards and backwards, effected by the external pterygoid muscles; when one of these muscles alone contracts, the forward movement of the chin is changed to a forward and lateral movement by the rotation of the jaw around the opposite condyle as a pivot; after being moved forward the jaw is retracted by the horizontal fibres of the temporal muscle; (3) grinding movements effected by a combina tion of the various muscles already mentioned. That the tongue and cheeks are not frequently nipped be tween the teeth is an indication of the perfect co-ordination of a very complex series of movements. The controlling sensations are chiefly conveyed along the fibres of the fifth cranial nerve. The motor fibres of the same nerve supply the chief but not all the muscles of mastication. After the food has been reduced to a proper consistency by the combined influence of the mechanical movements of the jaws, tongue, and cheeks, and the action of the saliva, it is rolled into a mass or bolus ready for swallow ing. The bolus is pushed on the dorsum of the tongue, which is hollowed into a shallow trough to receive it. It is easy to conceive how this shape of the tongue is brought about when we consider that the tongue is provided with vertical fibres, with horizontal antero-posterior fibres, and gland of neck; 3, submaxillary and other salivary glands; 4, J&quot;fe ular and subclavian veins; 5, situation of t i,ymus and thyroid glands; 6, entrance of thoracic duct into left subclavian vein, near the jugular; 7, left auricle; Bright auricle _; o, left ventricle; 10, right ventricle; 11, gall-blad- der; 12, vena portae; 13, mes- enteric glands; 14, lymphatic vessels; 15i lactea i vessels; ie, branches of the portal vein; 17,
 * &amp;gt; parotid gland; 2, salivary