516 PHYSIOLOGY CHAP. 



them. With direct microscopic observation of the mesenteric 

 lymphatics of small mammals, again, it is possible to follow the 

 slow centripetal movements of the lymph by the motion of the 

 leucocytes which it contains. 



In this movement of the lymph, as in the analogous case of 

 the blood, we have to determine the mechanical factors by which 

 it is produced, its velocity and pressure, and its variations under 

 given conditions. 



It is a fact that the lymphatic vessels are under a certain 

 degree of tension, i.e. they support a certain amount of pressure 

 which dilates them (Ludwig and Noll). In a lymphatic of the 

 horse's neck, the pressure is 10-20 rnm. of water (Weiss). 

 Since fluids moving in a tube always proceed from higher to lower 

 pressure, we must assume (even if it cannot be directly proved) 

 that the pressure is maximal at the roots of the lymphatics ; that 

 it gradually falls from the branches to the lymphatic vessels ; that, 

 lastly, it is minimal at the point at which the thoracic duct opens 

 into the left subclavian, and the right lymphatic trunk into 

 the right subclavian. A priori the velocity of movement of 

 the lymph should decrease from the greater trunks to the more 

 peripheral branches in proportion as the area of the current- 

 bed widens. Experimentally, however, it is found that even in 

 the larger lymphatic vessels, e.g. the lymphatic trunk of the 

 horse's neck, the velocity of movement is very low ; according to 

 Weiss it equals 250-300 mm. per minute. 



What is the origin of the vis a tergo which produces the 

 centripetal movement of the lymph, and is sufficient in man to- 

 overcome the force of gravity from the extreme end of the lower 

 limbs to the height of the venous vessels of the neck ? In the frog 

 and other amphibia, reptiles and fishes, the lymph hearts which 

 beat rhythmically, and which, by their muscular structure and 

 function, present many analogies with the blood heart, are 

 undoubtedly of great importance to the lymph flow. The frog is 

 provided with four lymph hearts : the two (posterior) sacral hearts 

 situated at the sides of the coccyx, being covered only by a delicate 

 aponeurosis and by the skin, can be seen beating even before they 

 are dissected ; the two (anterior) axillary hearts are covered by the 

 scapula. The sacral hearts carry away the lymph from the 

 lymphatics that accompany the sciatic vein ; the axillary hearts- 

 that of the vessels coining from the head and anterior limbs. 

 In other amphibia, reptiles and fishes, there are only two lymph 

 hearts. Without entering into their mode of functioning, we 

 can see that since the frog is poorly provided with regular 

 lymphatic vessels, and in compensation has a copious supply of 

 large sinuses and lymph sacs, the four hearts represent so many 

 pumps necessary for promoting the flow of lymph from the said 

 sacs and sinuses. 



