14 BLOOD AN INTERNAL MEDIUM. [BOOK i. 



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 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 blood vessel. 



In most tissues, as in muscle for instance, the capillary net- 

 work is so close set and the muscular fibre lies so near to the 

 blood vessel that the lymph between the two exists only as a very 

 thin sheet ; but in some tissues as in cartilage the blood vessels 

 lie on the outside of a large mass of tissue, the interchange 

 between the central parts of which and the nearest capillary blood 

 vessel 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 no, that the changes 

 are effected by means of the lymph. The blood may thus be 

 regarded as an internal medium bearing the same relations 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. Moreover 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 

 characters of the blood must be for ever 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 considera- 

 tion. 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. 



