84 ELEMENTARY PHYSIOLOGY less. 



anatomical components, the cells and tissues, of every 

 part of the body lie outside the blood-vessels. It is 

 therefore clear that the tissues are everywhere separated 

 from the blood by at least the thickness of the walls of 

 the vessels, and in any case cannot draw the nutriment 

 they require directly from the blood, since they are 

 nowhere in direct contact with it. Neither can they, 

 for the same reason, discharge the waste they are always 

 producing directly into the blood for its removal as a 

 preliminary to its excretion. Both these difficulties are 

 however got over by the fact that « ■portion of the fluid 

 part of the blood is continuall]/ exiiding through the icalls of 

 the capillaries into tlie neiglibouring tissues, taking with 

 it the nutriment necessary for each tissue and providing 

 a fluid connection between the tissue and the blood across 

 which the waste from the tissues can be returned into the 

 blood. The fluid which thus exudes is called lymph,' 

 and may be regarded as a sort of " middleman " between 

 the blood on the one hand and the tissue on the other. 

 But if now this lymj)!! is to be thoroughly efficient as a 

 nutriment for the tissues it should presumably contain 

 more food material than the tissues actually require as an 

 average, and it must therefore be an economy to the body 

 if the Ij'mph, after having served the needs of the tissues, 

 is gathered up again and returned to the blood for further 

 use. Now this is exactly wliat does take place, and tlie 

 means for ensuring the return of the lymph to the blood- 

 vessels are as follows. 



Besides the capillary network and the trunks connected 

 with it which constitute the blood-vascular system, all 

 parts of the body which possess blood cajiillaries also 

 contain another set of what are termed lymph-capil- 

 laries, mixed up with those of the blood-vascular system, 

 but not directly communicating with them, and, in 

 addition differing from the blood -capillaries in being 



1 The mode of formation, composition and properties of lymph are 

 dealt "with in Lesson III. 



