CHAP. iv. THE INTESTINAL VILLI. 273 



tube, and transferred to any or every organ of the body where they 

 may be required, no matter for what physiological purpose. Such 

 means are afforded by the blood, and by the blood alone. It is an 

 established fact that most of the absorption of nutrient substances 

 contained in the food takes place from the chyme in the small intestine. 

 To understand how this absorption is effected it is necessary to inquire 

 into the structure of the small intestine and to see what facilities the 

 arrangement of its tissues offers for the accomplishment of this 

 purpose. 



Of the several coats or layers which make up the small intestine the 

 innermost one is of chief interest in this connection ; it is called the 

 mucous membrane of the intestine and it is beset with numerous small 

 simple glands which secrete the succus entericus, but of which no 

 further mention need be made. This internal lining membrane is also 

 furnished with innumerable small outgrowths which impart to it a 

 somewhat villous or velvety appearance, and each little process is 

 appropriately called a villus. The villi are almost microscopic in size, 

 and they are so abundant and close-set as to confer upon the free 

 surface of the mucous membrane an appearance like that of the pile of 

 velvet. The structure of a villus is somewhat complex ; its outer part 

 consists of a coat of delicate thin-walled cells forming what is called 

 an epithelium. Along the middle of the inside space of the villus 

 there extends a more or less branched thin-walled tube, the various 

 parts of which originate blindly within the villus and coalesce into 

 one main tube which passes out of the villus at its base and finds its 

 way into the deeper walls of the intestinal canal. This narrow tube is 

 similar in origin and structure to numerous other tubes which are to 

 be found arising spontaneously in nearly all parts of the body, and are 

 known as lymphatic capillaries, but, for a reason that will be presently 

 mentioned, the lymphatics of the small intestine are distinguished 

 under the name of lacteals. Between the lacteal and the epithelial 

 wall of the villus there is a magnificent network of delicate, narrow, 

 thin-walled blood-vessels (capillaries), and at the base of each villus a 

 minute arte^ enters and breaks up into the capillaries which sub- 

 sequently coalesce so that their contents are poured into one or two 

 equally minute veins which leave the villus at its attached or basal end. 

 It is not difficult then to imagine the structure of a villus : the central 

 or axial part is occupied by the lacteal, this is quite surrounded by a 

 network of blood capillaries, which, in its turn is completely enveloped 

 by a sheath of epithelium, enclosing the inner structures like a thimble 

 does the tip of the finger (fig. 66, page 274). 



The importance of the villi will be appreciated when it is stated that 

 it is through their agency that the nutrients of the food are abstracted 

 from the chyme of the small intestine. Substances in solution, such 

 as peptones, carbohydrates, and salts, pass readily through the 

 epithelium of the villus and through the delicate walls of the blood 

 capillaries into the blood itself, and so leave the villus by the little veins 

 that pass away at the deeper end. Hence, the blood that leaves the walls 

 of the intestine by the intestinal veins differs in composition from that 



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