72 APPLIED PHYSIOLOGY 



sewage Blood is thus an epitome of the results of the 

 metabolic exchange between the organs and tissues. It 

 contains within itself representatives of all the soluble 

 constituents which play a part in the drama of cell life. 

 If any substance is being produced in excess it will be 

 found in the blood; if anything is in defect it is the 

 blood which will show it. 



Not only is the blood the great medium of exchange : it 

 is also in a sense an organ which has had entrusted to it 

 one of the most important of all functions in the com- 

 munity of cells which we call the body that, namely, of 

 defence. As pathologists penetrate more deeply into an 

 understanding of the means by which we are protected 

 from disease, these defensive functions of the blood 

 assume an ever-increasing importance, and the most 

 promising, though the most complicated, chapter in 

 modern bacteriology is that which deals with the anti- 

 toxic and bactericidal properties of the blood. 



Add to all this the comparative accessibility of the 

 blood and the ease with which many of its changes can 

 be studied, even in the living subject, and one does not 

 wonder that ' haematology ' has assumed such a large 

 place in latter-day medicine. 



We have spoken of the blood as a fluid medium of 

 exchange, but that is only one aspect of it. In virtue 

 of the fact that it contains living cells, the blood is also 

 to be regarded as a tissue. Even here it is unique, for 

 the blood is a peripatetic tissue free from nervous control. 

 If in its capacity as a fluid it is to be regarded as a 

 mirror of metabolism, so in its quality as a tissue the 

 blood may be considered as a reflection of the mature 



