CHAr. VI.] TBE ('AT\i ALIMENTARY SYSTEM. 181 



The inner surface of the small intestine does not form transverse 

 folds, but is clothed with a velvet-like lining made up of a multitude 

 of very fine, short, closely-set filaments or r////. These filaments 

 are jDrolongations of the corium, invested with columnar epithelium, 

 and contain blood-vessels, and a central vessel which is not a blood- 

 vessel, but is called a lacteal. The lacteals open j^roximally into 

 vessels belonging to the same category as they themselves do, and 

 which lie in the submucous areolar tissue. 



The lining of the small intestine is also beset with glands. The 

 most noticeable of these are aggregations of glands, which aggre- 

 gations go by the name of Pojer^s palcJics. There are in the small 

 intestine some six or seven of such patches, more or less narrow and 

 elongated, especially the one at the posterior end of the ileum, which 

 is clothed with villi like other parts of the intestine. Each patch is 

 made up of a number of glands, or vesicles, smaller than a pin's 

 head, composed of connective tissue, and containing a whitish fluid 

 with nucleated cells. 



Besides these structures, small glands of Liehorlciihn (like the 

 simple glands of the stomach) abound between the villi, and there 

 are other glands called Briiner's glands, which are branching 

 structures, most numerous in the duodenum. 



The small intestine is composed (1) of an outer or serous coat ; 

 (2) of two muscular layers ; (3) of an areolar or submucous coat ; and 

 (4) of the mucous lining. The serous coat is the peritoneal invest- 

 ment of the intestine, which is continuous with the two layers of the 

 mesentery by which it is suspended. It is wanting in part of the 

 duodenum. Of the muscular coats, the thinner external layer is 

 formed of longitudinal fibres, while in the thicker, inner layer they 

 are arranged circularly at right angles to the long axis of the tube. 

 The submucous coat is a layer of loose substance of areolar tissue 

 with fine elastic fibres, amdist which the blood-vessels ramify, and 

 subdivide before entering the innermost or mucous coat. 



The FUNCTION of the small intestine is, like that of the stomach, 

 partly mechanical and partly solvent. The successive contractions, 

 from before backwards, of its muscular walls tend to drive the 

 contents towards the large intestine. Such motion is, like the 

 similar movements of the fibres of the stomach, called peristaltic 

 action. This form of movement is also spoken of as the vermicular 

 motion of the intestine, and if the animal be suddenly killed, and its 

 abdomen opened, the peristaltic action will be seen still taking place, 

 and giving to the intestine an appearance as of so many crawling 

 worms — whence the term "vermicular." 



The process of digestion is further aided, as we shall hereafter see, 

 by the products of the pancreas and liver, and also somewhat by 

 the secretions of the crypts of Lieberklihn's and of Bruner's glands. 

 The former secrete the intestinal Juice proper, which is colourless, 

 and seems to be a form of mucus. 



The chyme of the stomach, having been modified by the action of 

 all these secretions, changes into what is called chyle, the secretion 



