ABSORPTION. 207 



diameter of a lymph-vessel varies from i to 2 mm. After the lymph- vessels 

 have emerged from the lymph-capillaries they acquire three distinct coats, 

 each of which possesses definite histologic features. 



The internal coat is composed of a delicate lamina of longitudinally dis- 

 posed elastic fibers covered with a layer of flattened nucleated endothelial 

 cells with wavy outlines. 



The middle coat consists of white fibrous tissue arranged longitudinally 

 and of non-striated muscle and elastic fibers arranged transversely. 



The external coat consists of practically the same structures, though the 

 muscle-fibers are longitudinally disposed. 



The lymph-vessels are provided with valves which are so numerous and 

 located at such short intervals as to give the vessels a beaded appearance. 



FIG. 87. LYMPH- VESSELS AND LYMPH-NODES OF THE HEAD AND NECK. 



These valves are arranged in pairs and consist of two semi-lunar folds with 

 their concavities directed toward the larger vessels. They are formed by a 

 reduplication of the lining membrane, which is strengthened by fibrous tissue 

 derived from the middle coat. 



Lymph-nodes, or glands. In their course toward the thoracic duct the 

 lymph- vessels pass through a number of small pisiform bodies termed lymph- 

 nodes or glands. These are exceedingly abundant in some situations, as the 

 cervical, axillary, and inguinal regions, and the abdominal cavity. As the 

 lymph- vessels approach a gland they divide into a number of branches before 

 entering it, known as the afferent vessels. From the opposite side of the 



