THE LYMPH AND LYMPHATIC SYSTEM 337 



the muscles and of the body generally. The valves, 

 like those of the veins, prevent the flow of the lymph 

 backwards, but allow it to pass forward towards the 

 heart. This is shown by the examination of a narcotized 

 mammal (killed immediately after the examination has 

 been made). A glass tube is placed in the thoracic 

 duct, and about a dozen drops of lymph (which would have 

 been delivered into the great vein) pass from it in a 

 minute. If, however, the animal's legs are moved, as 

 though in running, or if " massage " is applied to the 

 limbs — the pressure being directed from the extremities 

 towards the heart — then a greatly increased flow of 

 lymph is observed, as much as sixty drops in a minute ! 

 This is the chief explanation of the value to our health 

 of exercise, and also of the importance of " massage " as 

 a treatment in disease. Either exercise or massage 

 entirely revolutionizes the rate of fl' v of the lymph, 

 quickening it so greatly that the physiological effect on 

 the general chemical processes going on in the body 

 cannot fail to be most important. 



Curiously enough, whilst mammals have to depend 

 entirely on pressure and exercise for anything but the 

 slowest flow of the lymph, the cold-blooded vertebrates, 

 fish, amphibia and reptiles (and even some birds), have 

 remarkable, rhythmically contracting, muscular sacs, 

 which pump the lymph from large lymph-vessels into 

 large veins, and are called " lymphatic hearts." The 

 eel and other fish have them in the tail, but they are 

 best seen in the common frog. There is an anterior 

 pair, one under each shoulder-blade, and another pair, 

 one on each hip. Each opens at one end into a large 

 "collecting" lymph-vessel, and at the other end into a 

 large vein. They " beat " like a heart, but do not keep 

 time with one another. Their muscular walls are formed 

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