528 THE TISSUES. 



tics of the villi are usually called lacteals. These traverse the axis 

 of the villus, commencing in a caecal and frequently enlarged end. 

 (Fig. 356.) They have a much greater diameter than the capilla- 

 ries, and, according to Professor Briicke, are mere excavations in 

 the villi without walls, while the true chyliferous vessels commence 

 in the deeper parts of the membrane. In some broad villi, two 

 lacteal cavities, a lon^ and a short, appear. 



The smooth muscular fibres of the villi are arranged longitudinally, 

 forming a thin layer, not always distinct in man, placed centrally 

 around the lacteals. They produce contractions, and thus influence 

 the propulsion of the chyle and the venous blood in the villi. 

 Nothing is known of any nerves in them. 



Function of the Villi. The villi are the principal agents of ab- 

 sorption of the nutritive elements resulting from the digestion of 

 the food. But Briicke shows that absorption also occurs on the 

 surfaces between them, and particularly from between the glands 

 of Lieberkiihn. It is generally asserted that the lacteals alone 

 absorb fat, while the minute bloodvessels absorb the other elements. 

 Bruch, however, found that the bloodvessels absorb fat as well as 

 the lacteals; the former sometimes being half filled with fat, instead 

 of blood alone. Both these observers also show that the epithelium 

 of the villi is not cast off during normal digestion, as stated by Mr. 

 Goodsir. Briicke asserts that the epithelial cells are mere tubes, 

 closed externally by a layer of mucilaginous substance easily per- 

 meable by fluids, and that the fat therefore finds an easy admission 

 into them, and to the surface and into the substance of the proper 

 villus itself, in the form of oil-drops. This observation needs con- 

 firmation. 



In cholera, the epithelium of the villi, and sometimes of the 

 whole intestine, is thrown off. 



II. The glands of the small intestine are of two kinds, the tubular 

 $and the racemose. Certain closed follicles are, however, also to be 

 described in this connection. 



1. The tubular, or Lieberklihn's, glands are distributed over the 

 whole small intestine, as straight, narrow caeca (Fig. 357), extending 

 completely through the mucous membrane, and occupying almost 

 all the space left between the villi; and, in a vertical section (Fig. 

 359 and 362), resembling palisades. They are, however, not found 

 over the centre of the closed follicles, as will be seen. Their length 

 equals the thickness of the mucous membrane (g 1 ^ to ^\ of an inch); 



