256 ELEMENTARY PHYSIOLOGY LEsa 



covering the villi differ in shape from those lining the 

 glands (Fig. 82, B, C). 



At irregular intervals along the mucous membrane the 

 lymphoid tissue between the glands is developed into 

 small rounded masses called "solitary glands" or 

 "solitary follicles." These closely reseml)le the 

 glandular substance of the alveoli at the cortex of a lym- 

 phatic gland (p. 90), and are similarly crowded with leuco- 

 cytes. In parts of the small intestine, more particularly 

 in the ileum, groups of these follicles are found packed 

 closely together ; they are then known as " agminnted 

 glands," or Peyer's patches; these do not occur in 

 the large intestine. 



At the commencement of tlie duodenum are certain 

 small racemose glands, called the glands of Brunner, 

 whose ducts open into the intestine. Their function 

 seems to be quite unimportant. 



12. The Structure of the Villi.— A villus, as already 

 explained, is a projection into the cavity of the intestine 

 of the tissue between the glands of Lieberkiihn, and is 

 covered by an epitlielium continuous with that which 

 lines the.se glands. The average length of a villus is about 

 •5 — "7 of a millimetre ( Jq - 5^5- of an inch). Running up the 

 centre or axis of the villus is a relatively large lymphatic 

 vessel which ends blindly at the summit of the villus, but 

 at its base opens into the lymphatics in the submucous 

 tissue (Fig. 82, /). This central lymphatic is called a 

 lacteal. Lying around the lacteal and parallel to it, are 

 a few small fibres of unstriated muscle derived from the 

 muscularis mucosae, which is a thin layer of un- 

 striated muscle in the mucous membrane, lying next to 

 the submucous coat (Fig. 82, m.m) ; outside these 

 again, close under the epithelium of the villus, is a net- 

 work of capillaries (Fig. 82, c), which receive blood from 

 an artery in the submucous layer and return it by a small 

 vein to the veins of the same layer. 



All the space left in between the several structures so 



