SMALL INTESTINE. 321 



lay the food somewhat in its passage, since it must collect in 

 the hollows between them, and so be longer exposed to the 

 action of the digestive liquids. Examined closely with the 

 eye or, better, with a hand lens, the mucous membrane of 

 the small intestine is seen to be not smooth but shaggy, be- 

 ing covered everywhere (both over the valvulae conniventes 

 and between them) with closely packed minute processes, 

 standing up somewhat like the ' pile'' on velvet, and known 

 as the villi. Each villus is from 0.5 to 0.7 millimeters 

 {gV to -g^ inch) long ; some are conical and rounded, but 

 the majority are compressed at the base in one diameter 

 (Fig. 99). In structure a villas is somewhat complex. 

 Covering it is a single layer of columnar epithelial cells, be- 

 neath which the villus may be regarded as made up of a 



FIG. 98. - A portion of the small intestine opened to show the valvulce conni- 

 rentes. 



framework of connective tissue supporting the more essen- 

 tial constituents. Near the surface is an incomplete layer 

 of plain muscular tissue, continuous below with a muscular 

 layer found on the deep side of the mucous membrane. In 

 the centre is an offshoot of the lymphatic system; some- 

 times in the form of a single vessel with a closed dilated 

 end, and sometimes as a network formed by two main ves- 

 sels with cross-branches. During digestion these lym- 

 phatics are filled with a milky white liquid absorbed from 

 the intestines and they are accordingly called the lacteals. 

 They communicate with larger branches in the submucous 

 coat which end in trunks that pass out in the mesentery to 

 join the main lymphatic system. Finally, in each villus, 



