BOOK I. 



BLOOD, THE TISSUES OF MOVEMENT, THE VASCULAR MECHANISM, 



CHAPTER I. 



BLOOD. 



13. THE several tissues are traversed by minute tubes, the capillary 

 bloodvessels, 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 occu- 

 pied by the elements of the tissue, which consequently lie outside the capil- 

 laries. 



The blood flowing through the capillaries consists, under normal condi- 

 tions, of an almost colorless fluid, the plasma, in which are carried a num- 

 ber of bodies, the red and the white corpuscles. Outside the capillary walls, 

 filling 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 colorless 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 capil- 

 laries 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 through 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 mid- 

 dleman, 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 bloodvessels, 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 car- 

 ries to the tissue the material which the tissue needs for building itself up 

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

 from the tissue to the blood carries into the blood certain of the products of 

 the chemical changes which have been taking place in the tissue, products 

 which may be simple waste, to be cast out of the body as soon as possible, 

 or which may be bodies capable of being made use of by some other tissue. 



A third stream, that from the lymph lying in the chinks and crannies 

 of the tissue along the lymph channels to the larger lymph vessels, carries 

 away from the tissue such parts of the material coming from the blood as 



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