1636 



HUMAN ANATOMY. 



FIG. 1385. 



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 involuntary muscle. The reticular tissue constituting the villus 

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

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

 one to three small arteries, derived 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-vessel occupying the villus 

 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 

 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 (Ebner). 



The valvulse conniventes 

 (plicae circulates ), 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 eontain 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 turn-. Their ends, usually simple, may In- bifurcated. Smaller folds, more or 

 less effaeeable. run obliquely as offshoots from the larger ones. The valves arc much 

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

 they mav be entirely .lUent in localities in which the folds are feebly developed. 

 Sin-reeding tin- first part of the du< .deinnn, the \alvula- ronniventes 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 

 sin. ill. -i and farther apart, so that the distance between them considerably exceeds 

 their height. They also become in. .re H'fareable. and finally very much so. In 



Gland 



Villus 



Surface view of mucous membrane of jejunum, showing villi 

 and orifices of glands. X 13. 



