Mammalia: Visceral Systems 



633 



Fig. 180. The lymphatics of the human scrotum, showing the transition from 

 capillaries to vessels with valves (a, a, a). (After Teichmann. Courtesy, Jackson: 

 "Morris' Human Anatomy," Philadelphia, The Blakiston Company.) 



Veins also contain valves permitting flow only toward the heart. 

 They are especially numerous in veins of the body-wall and append- 

 ages, less so in the visceral veins, and entirely lacking in veins of the 

 brain and spinal cord which are not subject to external muscular 

 pressure. Acceleration of flow of lymph into the veins and flow of 

 venous blood toward the heart is a vitally important by-product of 

 vigorous muscular exercise. In birds the lymph-vessels contain valves, 

 but they are less numerous than in mammals. In the absence of lymph- 

 hearts, it is probable that the flow of lymph in birds, as in mammals, is 

 partly dependent on compression of the vessels by adjacent muscles. 



Lymphoid tissue, whose main function is production of lympho- 

 cytes (p. 329), exists in all vertebrates, occurring in small masses 

 widely distributed over the body. In Amniota it also becomes locally 

 aggregated to form definite organs or "lymph-glands" (cytogenic, not 

 secretory glands; see p. 310). Doubtfully present in reptiles, a few 

 lymph-glands occur in birds, but in mammals they are numerous, 

 usually definitely localized, and some attain considerable size — e.g., 

 the originally misnamed "pancreas Aselli" in the mesentery of the 

 cat. The larger lymph-glands occur in the cervical, axillary, and 

 inguinal regions, where they are either subcutaneous or lie between 

 muscles. There are numerous lymph-glands also on the walls of the 

 thoracic and abdominal viscera and in the mesenteries. The glands 



