26 BLOOD. 



are not taken up by the tissue itself and such parts of the material coming 

 from the tissue as do not find their way into the bloodvessel. 



In most tissues, as in muscle for instance, the capillary network is so 

 close set and the muscular fibre lies so near to the bloodvessel that the 

 lymph between the two exists only as a very thin sheet; but in some tis- 

 sues, as in cartilage, the bloodvessels lie on the outside of a large mass of 

 tissue, the interchange between the central parts of which and the nearest 

 capillary bloodvessel is carried on through a long stretch of lymph passages. 

 But in each case the principle is the same ; the tissue, by the help of lymph, 

 lives on the blood; and when in succeeding pages we speak of changes 

 between the blood and the tissues, it will be understood, whether expressly 

 stated so or not, that the changes are effected by means of the lymph. The 

 blood may thus be regarded as an internal medium bearing the same rela- 

 tions to the constituent tissues that the external medium, the world, does to 

 the whole individual. Just as the whole organism lives on the things 

 around it, its air and its food, so the several tissues live on the complex 

 fluid by which they are all bathed, and which is to them their immediate 

 air and food. 



All the tissues take up oxygen from the blood and give up carbonic acid 

 to the blood, but not always at the same rate or at the same time. More- 

 over, the several tissues take up from the blood and give up to the blood 

 either different things or the same things at different rates or at different 

 times. 



From this it follows, on the one hand, that the composition and charac- 

 ters of the blood must be forever varying in different parts of the body and 

 at different times ; and on the other hand, that the united action of all the 

 tissues must tend to establish and maintain an average uniform composition 

 of the whole mass of blood. The special changes which blood is known to 

 undergo while it passes through the several tissues will best be dealt with 

 when the individual tissues and organs come under our consideration. At 

 present it will be sufficient to study the main features which are presented 

 by blood brought, so to speak, into a state of equilibrium by the common 

 action of all the tissues. 



Of all these main features of blood the most striking, if not the most 

 important, is the property it possesses of clotting when shed. 



THE CLOTTING OF BLOOD. 



14. Blood, when shed from the bloodvessels of a living body, is per- 

 fectly fluid. In a short time it becomes viscid, this viscidity increasing rap- 

 idly until the whole mass of blood under observation becomes a complete 

 jelly. The vessel into which it has been shed can at this stage be inverted 

 without a drop of the blood being spilt. The jelly is of the same bulk as 

 the previously fluid blood, and if carefully shaken out will present a com- 

 plete mould of the interior of the vessel. [Fig. 2.] If the blood in this 

 jelly stage be left untouched in a glass vessel, a few drops of an almost 

 colorless fluid soon make their appearance on the surface of the jelly. 

 Increasing in number, and running together, the drops after a while form 

 a superficial layer of pale, straw-colored fluid. Later on, similar layers 

 of the same fluid are seen at the sides and finally at the bottom of the jelly, 

 which, shrunk to a smaller size and of firmer consistency, now forms a clot 

 or crassamentum, floating in a perfectly fluid serum. [Fig. 3.] The shrinking 

 and condensation of the clot, and the corresponding increase of the serum, 

 continue for some time. The upper surface of the clot is generally slightly 

 concave. A portion of the clot examined under the microscope is seen to 



