104 HUMAN PHYSIOLOGY. 



portions of the blood through their delicate walls, but are also engaged in the 

 reabsorption of this transudate, as well as in the absorption of new materials 

 from the alimentary canal. The extensive capillary network which is formed 

 by the ultimate subdivision of the arterioles in the submucous tissue and 

 villi of the small intestine forms an anatomic arrangement well adapted for 

 absorption. It is now well known that in the absorption of the products of 

 digestion the blood-capillaries are more active than the lymph-capillaries. 



Lymph-vessels. These constitute a system of minute, delicate trans- 

 parent vessels, found in nearly all the organs and tissues of the body. Having 

 their origin at the periphery in the lymph-capillaries and spaces, they rapidly 

 converge toward the trunk of the body and empty into the thoracic duct. In 

 their course they pass through numerous small ovoid bodies, the lymphatic 

 glands. 



The lymph-vessels of the small intestines//^ lacteals arise within the 

 villous processes which project from the inner surface of the intestine through- 

 out its entire extent. The wall of the villus is formed by an elevation of the 

 basement membrane, and is covered by a layer of columnar epithelial cells. 

 The basis of the villus consists of adenoid tissue, a fine plexus of blood-vessels, 

 unstriped muscle-fibers, and the lacteal vessel. The adencid tissue consists 

 of a number of intercommunicating spaces, containing leukocytes. The 

 lacteal vessel possesses a thin but distinct wall composed of endothelial 

 plates, with here and there openings which bring the interior of the villus 

 into communication with the spaces of the adenoid tissue. 



The structure of the larger vessels resembles that of the veins, consisting 

 of three coats: 



1. External, composed of fibrous tissue and muscle fibers, arranged longi- 

 tudinally. 



2. Middle, consisting of white fibers and yellow elastic tissue, nonstriated 

 muscle-fibers, arranged transversely. 



^3. Internal, composed of an elastic membrane, lined by endothelial cells. 



Throughout their course are found numerous semilunar -valves, opening 

 toward the larger vessels, formed by a folding of the inner coat and strength- 

 ened by connective tissue. 



Lymph Glands. The lymph glands consist of an external capsule com- 

 posed of fibrous tissue which contains non-striped muscle-fibers; from its 

 inner surface septa of fibrous tissue pass inward and subdivide the gland- 

 substance into a series of compartments, which communicate with one 

 another. The blood-vessels which penetrate the gland are surrounded by 

 fine threads, forming a follicular arrangement, the meshes of which contain 

 numerous lymph-corpuscles. Between the follicular threads and the wall 



