PHYSIOLOGICAL 349 



acid. Lastly, carbon dioxide is also carried, to a considerable extent, 

 by the haemoglobin in the corpuscles themselves. 



A problem which has yet to be considered is the balance main- 

 tained by the blood acidity and alkalinity. This is important; in 

 the first place because it is probably an increase in acidity of the 

 blood which causes the "respiratory centre" of the brain to send 

 nervous messages to the lungs, commanding them to expand and 

 draw in more air; this increased acidity being due to an increase of 

 the amount of free carbon dioxide in the blood. It should also be 

 mentioned that the blood has a remarkable "buffer" action, or power 

 of counteracting any changes in its alkalinity, which, though 

 varying in the cycle between venous and arterial blood, remains 

 singularly constant at any one point in the circulation. 



We find, then, as L. J. Henderson has shown, that the blood is 

 a complex physico-chemical system, in which there are at least six 

 variable factors: the oxygen tension, the carbon dioxide tension, 

 the amount of combined oxygen (oxyhsemoglobin), the amount of 

 combined carbon dioxide (in various forms), the distribution of 

 chlorides between corpuscles and plasma, and the alkalinity or 

 hydrogen-ion-concentration of the plasma. All these factors vary 

 together; and if any two are arbitrarily fixed, the other four can be 

 accurately calculated, according to definite laws of physico-chemical 

 equilibria. 



6. Salts of the Blood and Salts of the Sea. — Of much 

 biological interest is the correspondence between the salts of the 

 blood and the salts of the sea. Both in the nature of these salts and 

 in their relative proportions there is a remarkable likeness between 

 blood and sea-water, as has been worked out by Quinton and 

 Macallum. The correspondence is greater when account is taken of 

 the change in the composition of the sea — towards increased salinity 

 — since blood-possessing animals were first evolved in Cambrian 

 times. To start with, the specific gravity of the two fluids was 

 probably the same. "We cannot shake off the lien the past has 

 upon us; when our head throbs we may hear the primeval ocean 

 breaking on the Cambrian shore." 



7. Immunity. — One of the most remarkable properties of the 

 blood is the power the serum has of forming specific substances to 

 combine with foreign substances introduced into the body. Sub- 

 stances which provoke this reaction are called "antigens". Any 

 soluble protein not normally present in the blood may act as an 

 antigen; but it is probable that only proteins can do so. The 

 substance which the serum forms in response to the introduction 

 of an antigen is called an anti-body, and the reaction between 

 antigen and anti-body may be any one of various different types. 

 If the antigen is soluble, it will be precipitated by the anti-body. If 



