12 BLOOD AN INTERNAL MEDIUM. [BOOK i. 



oxygen) which they need for the manifestation of energy or for 

 the storing up of differentiated material, and return to it the waste 

 products resulting from their activity. All over the body every- 

 where there is so long as life lasts a double current, here rapid, 

 there slow, of material from the blood to the tissues, and from the 

 tissues to the blood. 



It, together with lymph (whether in the lymph-canals or in the 

 interstices of the tissues), may, as Bernard has suggested, 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. 



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 or 

 coagulating when shed. 



