CHAPTER I. 



BLOOD. 



§ 13. The several tissues are traversed by minute tubes, — the 

 capillary blood vessels, — to which blood is brought by the arteries, 

 and from which blood is carried away by the veins. These 

 capillaries form networks the meshes of which, differing in form 

 and size in the different tissues, are occupied by the elements of 

 the tissue which consequently lie outside the capillaries. 



The blood flowing along the capillaries consists, under normal 

 conditions, of an almost colourless fluid, the plasma, in which 

 are carried a number of bodies, the red and the white corpuscles. 

 Outside the capillary walls, tilling up such spaces as exist between 

 the capillary walls and the cells or fibres of the tissue, or between 

 the elements of the tissue themselves, is found a colourless fluid, 

 resembling in many respects the plasma of blood and called 

 lymph. Thus all the elements of the tissue and the outsides of 

 all the capillaries are bathed with lymph, which, as we shall see 

 hereafter, is continually flowing away from the tissue along 

 special channels to pass into lymphatic vessels and thence into 

 the blood. 



As the blood flows along the capillaries certain constituents 

 of the plasma (together with, at times, white corpuscles, and 

 under exceptional circumstances red corpuscles) pass through 

 the capillary wall into the lymph, and certain constituents of the 

 lymph pass through the capillary wall into the blood within the 

 capillary. There is thus an interchange of material between 

 the blood within the capillary and the lymph outside. A similar 

 interchange of material is at the same time going on between the 

 lymph and the tissue itself. Hence, by means of the lymph acting 

 as middleman, a double interchange of material takes place between 

 the blood within the capillary and the tissue outside the capillary. 

 In every tissue, so long as life lasts and the blood flows through 

 the blood vessels, a double stream, now rapid now slow, is passing 

 from the blood to the tissue and from the tissue to the blood. 

 The stream from the blood to the tissue carries to the tissue 

 the material which the tissue needs for building itself up and 

 for doing its work, including the all-important oxygen. The 



