214 



THE INTI RNAL ENVIRONMENT OF THE BODY 



Part III 



B 



Fig. 12.13. A, The superficial lymph vessels of the thumb and finger. A small 

 part of the great network in which a balance of fluid is maintained with that in 

 the blood vessels and other tissues. B, Superficial lymph nodes in axil of arm 

 and throat region; both are incomplete but they suggest the prevalence of lymph 

 nodes. (Redrawn after Brash, ed.: Cunningham's Textbook of Anatomy, ed. 9. 

 New York, Oxford University Press, 1951.) 



behind it, by breathing movements, and the contractions of muscles; valves 

 keep it from going backward just as valves do in many of the veins. In 

 mammals there are no lymph hearts as there are in frogs. 



Blood Circulation in Mammals 



The blood vessels form a complete series of intercommunicating tubes. 

 The heart is an enlarged and sharply bent part of a tube protected by the 

 pericardial sac. The tubular shape of the heart can be seen clearly in the de- 

 velopment of the human embryo and other higher vertebrates, and in adult 

 fishes (Chap. 19). In fishes the heart is continuous at one end with the 

 arteries that carry blood away from it and at the other with veins that return 

 blood to it. Connecting the larger vessels are the microscopic capillaries 

 usually between arteries and veins, but in the hepatic portal system between 

 veins and veins. 



