212 TEXT-BOOK OF PHYSIOLOGY 



determined largely by the nature of the tissue in which they are found, it 

 is not always possible to separate one from the other. Their function, how- 

 ever, may be regarded as similar: viz.: the reception and collection of the 

 excess of lymph which has transuded through the walls of the blood-vessels 

 and its transmission onward into the regular lymph-vessels. 



Lymph-vessels. The lymph-vessels constitute a system of minute, 

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

 body, and take their origin from the lymph-capillaries and spaces above 

 described. From their origin they gradually converge toward the trunk 

 of the body, and finally empty into the thoracic duct. In their course they 

 anastomose very freely with adjoining vessels. The 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 pos- 

 sesses 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. 

 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 

 gland the lymphatics again emerge as efferent vessels to unite to form larger 

 trunks. A section of a gland shows that it consists of an outer dense cortical 

 and an inner soft pulpy medullary portion. Each gland is covered exter- 

 nally by a dense membrane of fibrous tissue containing in its meshes non- 

 striated muscle-fibers. From the inner surface of this membrane there pass 

 inward septa of connective tissue which, as they converge toward the center 

 of the gland, divide its outer zone into small conical compartments or 

 alveoli. When the septa reach the medullary portion, they subdivide and 

 form bands or cords which interlace in every direction and constitute a loose 

 meshwork the spaces of which communicate with one another and with the 

 alveoli. Within the meshes of this framework the proper gland substance 

 is contained. In the cortical compartments it is moulded into pear-shaped 

 masses; in the medullary meshwork it assumes the form of rounded cords 

 which are connected with one another. In both regions, however, it is 

 separated from the septa by a space termed a lymph sinus, through which 

 the lymph flows as it passes through the gland. The lymph sinus is crossed 

 by a network of retiform connective tissue which offers considerable re- 



