THE SMALL INTESTINE. 



395 



plexus myentericus of flat nerve-fibres, which will be described 

 farther on. The muscle-coat becomes gradually thinner as we 

 pass from the duodenum to the ileo-csecal valve. In the for- 

 mation of this thickened fold the longitudinal layer does not 

 participate. 



The unstriped muscle-cells have an average length of 0.255 

 mm., and are about 0.005 mm. broad. They are arranged in 

 bundles, surrounded by connective-tissue bands, with which 

 elastic elements are abundantly interwoven. 



The mucous membrane is thrown into folds, and is studded 

 with closely placed projections, called villi. The general di- 

 rection of these folds, the valvulce conniventes KerTtringii, is 

 parallel to the transverse course of the circular muscle-layer. 

 They run parallel to one another, or join at acute angles. 



The mill jut out into the lumen of the intestinal canal, as 

 variously shaped projections, of an average length of 0.04 0.6 

 mm., and an average breadth of 

 0.06 0.12 mm. In general their 

 form may be said to be conical or 

 cylindrical ; but we always en- 

 counter a great variety of shapes, 

 in accordance with the varying 

 states of contraction in the mus- 

 cularis mucosae. Each villus con- 

 sists of a large-meshed reticulum 

 of connective tissue, infiltrated, as 

 it were, with leucocytes, and con- 

 taining flattened corpuscles, which 

 resemble endothelial cells. One or 

 several spaces, situated in the cen- 

 tre of every villus, constitute the 

 origin of the lacteal tubes. Ac- 

 cording to Briicke, these chyle- 

 vessels are covered by thin, but 

 not continuous bundles of smooth muscle-fibres. Their walls 

 show only a single layer of ordinary endothelial cells, with 

 clear oval nuclei. The free surface of the villi, like that of the 

 stomach, is covered by a single layer of columnar epithelium. 

 Each cell presents, in the recent state, a finety striated hyaline 

 band at its unattached border. This structure has, at different 

 times, received various interpretations, and even now opinions 



Fio. 169. Soction of a villus from the 

 intestine of a rabbit : a, epithelium ; 6, 

 Btrouia ; c, central cavity. Verson. 



