GENERAL STRUCTURE OF THE BODY. 33 



ally during digestion, when fat is present. Here it is 

 called chyle. Otherwise it is generally a clear, trans- 

 parent and slightly opalescent fluid, which, owing to 

 the presence of fibrin, clots when drawn from the body 

 and allowed to stand. In fact, it resembles blood 

 plasma very closely in composition and, as it also con- 

 tains a certain number of corpuscles or leucocytes that 

 just correspond to the white corpuscles of the blood, 

 it is practically blood without the red corpuscles. 

 These leucocytes have considerable power of amoeboid 

 movement and are thought by some to play an im- 

 portant part in the absorption of food. 



Owing to intracapillary pressure, the lymph trans- 

 udes into the lymph spaces and bathes the tissues, being 

 carried away again by the lymphatics. The amount of 

 transudation is determined by the blood pressure the 

 greater the pressure, the greater the amount of trans- 

 udation and is increased by some organic action of the 

 cells in the walls of the vessels. In the process of transu- 

 dation a certain amount of solid matter goes through the 

 wall of the vessel and it is probable that certain protein 

 elements can be carried thus from the blood-vessels to 

 the lymphatics, though they do not pass through the 

 capillary wall as readily as other substances. Some 

 lymph is also probably formed by the action of the tis- 

 sues themselves, though the process is not understood. 



All muscular movements, active or passive, includ- 

 ing the respiratory movements, tend to drive the lymph 

 on its way by pressure, the valves of the vessels keep- 

 ing it from flowing back. Moreover, its flow is from the 

 capillaries to the veins or from a region of high pressure 

 to one of less pressure. There is probably also some con- 

 traction in the walls of the vessels themselves, and the 

 continual formation of lymph helps to drive it along. If 

 an obstruction to the circulation occurs, however, back- 

 pressure results and causes too great transudation. In 

 that event a limb becomes swollen, pale, and generally 

 cool. It pits on pressure, the pressure driving the lymph 



