THE VASCULAR MECHANISM. 239 



more and more feebly, fails to expel its contents into the aorta, and 

 finally the circulation may come to a standstill. Thus the nutrition of 

 the heart muscle is ultimately the most important feature of the 

 circulatory mechanism. 



SECTION VI. 



THE FORMATION OF LYMPH. 



Except in the spleen, and possibly the liver, the blood does not come 

 into direct contact with the cells of the tissues. It is separated from 

 them not only by the walls of the capillaries, but by a fluid called lymph 

 or tissue fluid, which lies between the capillaries and the tissue cells 

 themselves. From these spaces the lymph passes into narrow channels 

 (lymphatic vessels) lined by endothelial cells. These channels unite 

 and finally end in a single vessel, the thoracic duct, which opens into 

 the junction of the left jugular and subclavian veins, and conveys the 

 lymph from the greater part of the body into the blood stream. The 

 lymph from the right side of the head and neck, and the right fore-limb 

 passes into a vessel which opens into the junction of the right jugular 

 and subclavian veins. The lymph has been described by Foster as a 

 u middle man," since, on the one hand, it receives from the blood oxygen 

 and dissolved nutrient materials and passes them on to the tissue cells, 

 and, on the other hand, it receives from the tissue cells carbonic acid 

 and other waste products and returns them to the blood stream. The 

 interchange of material between the blood and the tissues takes place 

 by diffusion (p. 14), and in this way the tissues are nourished without 

 any increase necessarily taking place in the amount of tissue fluid. 



THE COMPOSITION OF LYMPH. 



Lymph can be collected by placing a cannula in the thoracic duct 

 of an animal, such as a dog or horse. If the animal has not been 

 recently fed, the lymph is a clear, colourless fluid having a specific 

 gravity of about 1015, and usually clots when allowed to stand. It 

 contains some lymphocytes, 4 to 5 per cent, of protein, the proteins 

 being the same as those in blood serum, and also various salts and 

 extractives. After a meal the lymph is milky in appearance, owing to 

 the presence of large numbers of minute fat globules. The fat is 

 derived from that taken in the food, which, after absorption from the 

 digestive tract, passes into the intestinal lymphatic vessels (lacteals). 

 In their course the vessels pass through the lymphatic glands, in which 

 lymphocytes are formed ; these enter the lymph stream and are carried 

 into the blood. 



