1636 



HUMAN ANATOMY. 



the mucous coat alone, and consist of a framework of the lymphoid stroma-tissue, 

 covered by columnar epithelium, which supports the absorbent vessel and the blood- 

 vessels, together with in\oluntary muscle. The reticular tissue constituting the villus 

 is condensed at the periphery, the existence of a dehnite limiting membrane being 

 assumed by some (J. Schaffer, Spalteholz, EbnerJ. Each villus is supplied by from 

 one to three small arteries, derixed from the vessels of the submucosa, which break 

 up into a capillary net-work lying beneath the peripheral layer of the stroma. The 

 blood is returned usually by a single vein which, beginning at the summit by the 

 confluence of capillaries, traverses the central parts of the villus and becomes trib- 

 utary to the larger venous stems within the submucous coat. 



The absorbent, chyle-vessel, or lacteal, as the lymph-x-essel occupying the \Hlus 

 is variously termed, lies near the centre of the projection, surrounded by the mus- 

 cular tissue and the blood-capillaries. While the slender cylindrical villi contain 

 only a single lymphatic, from .025-035 mm. in diameter, those of broader form often 

 contain two, three, or even more such vessels, which may communicate by cross- 

 channels. Their walls consist of a single layer of endothelial plates. The muscular 

 tissue within the villus, prolonged from the muscularis mucosae, forms a delicate 

 layer of slender fibre-cells, longitudinally disposed, which surround the central chyle- 

 vessel. Contractions of this tissue 

 Fig. 1385. shorten the villus and aid in propel- 



ling the emulsified contents of the 

 lymphatic. 



The presence of numerous oil- 

 droplets of considerable size within 

 the epithelial cells, as well as stroma, 

 of the villi during certain stages of 

 digestion has caused much specula- 

 tion as to their mode of entrance. 

 On histological grounds there is 

 good reason for assuming that a large 

 part of the fat particles seen within 

 the tissues gains access in a condition 

 either of solubility, saponification, 

 or exceedingly fine molecular sub- 

 division, the accumulations observed 

 within the tissues being due to sec- 

 ondary change (F2bner). 



The valvulae conniventes 

 (plicae circulares), within the duo- 

 denum and the jejuno-ileum, model 

 the mucous coat and greatly increase its secreting and absorbant surface; they 

 also retard the passage of the intestinal contents, thereby facilitating the diges- 

 tive processes. These transverse folds begin in the second part of the duodenum 

 and consist of duplicatures which involve not only the entire thickness of the 

 mucosa, but contain a central supporting projection of the submucous coat; 

 hence, while they may fall on their sides, they cannot, as a rule, be effaced by dis- 

 tention. The height of the folds, where well developed, rarely much exceeds i cm., 

 and towards the lower part of the jejunum is much less. The majority of the valves 

 do not extend more than two-thirds or three-fourths of the circumference of the 

 gut ; exceptionally, however, circular and spiral ones describe two or three com- 

 plete turns. Their ends, usually simple, may be bifurcated. Smaller folds, more or 

 less efTaceable, run obliquely as offshoots from the larger ones. The valves are much 

 larger on the attached side of the gut than on the free one ; in the latter position 

 they may be entirely absent in localities in which the folds are feebly developed. 

 Succeeding the first part of the duodenum, the valvulae conniventes are very numer- 

 ous and large, and so near together that in falling over any fold would come in con- 

 tact with the next one. Descending the small intestine, they gradually become 

 smaller and farther apart, so that the distance between them considerably exceeds 

 their height. They also become more ef?aceable, and finally very much so. In 



Gland^ 



Villus 



Surface view of mucous membrane of jejunum, showing villi 

 and orifices of glands. X 12. 



