116 HUMAN PHYSIOLOGY. 



As the shape, size, etc., of both lymph-spaces and capillaries are 

 determined largely by the nature of the tissues in which they are 

 contained, it is not always possible to separate the one from the other. 

 Their function, however, may be regarded as similar viz., the col- 

 lection of the lymph which has escaped from the blood-vessels, and 

 its transmission onward into the regular lymphatic vessels. 



The blood-capillaries not only permit the escape of the liquid 

 nutritive 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 exten- 

 sive capillary network which is formed by the ultimate subdivision 

 of the arterioles in the submucous tissue and villi of the small intes- 

 tine 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 

 transparent vessels, found in nearly all the organs and tissues of the 

 the body. Having their origin at the periphery in the lymph-capil- 

 laries 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 the lacteals arise 

 within the villous processes which project from the inner surface of 

 the intestine throughout its entire extent. The wall of the villus 

 is formed by an elevation of the basement membrane, and is cov- 

 ered 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 adenoid tissue consists 

 of a number of intercommunicating spaces, containing leukocytes. 

 The lacteal vessel possesses a thin but distinct wall composed of en- 

 dothelial 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 

 longitudinally. 



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

 striated muscle-fibers, arranged transversely. 



