THE BLOOD. 83 



atmosphere becomes equal. This condition is called a condition of equal 

 tensions. 



The condition may be understood by a simple illustration. A large 

 amount of carbonic acid gas is dissolved in a bottle of water by exposing 

 tha liquid to extreme pressure of the gas, and a cork is placed in the 

 bottle and wired down. The gas exists in the water in a condition of 

 extreme tension, and therefore exhibits a tendency to escape into the 

 atmosphere, in order to relieve the tension; this produces the violent 

 expulsion of the cork when the wire is removed, arid if the aerated water 

 be placed in a glass the gas will continue to be evolved until it has almost 

 entirely passed into the atmosphere, and the tension of the gas in the 

 water approximates to that of the atmosphere in which, it should be 

 remembered, the carbon dioxide is, naturally, in very small amount, 

 viz., .04 per cent. 



The oxygen of the blood does not obey this law of pressure. For if 

 blood which contains little or no oxygen be exposed to a succession of 

 atmospheres containing more and more of that gas, we find that the 

 absorption is at first very great, but soon becomes relatively very small, 

 not being therefore regularly in proportion to the increased amount (or 

 tension) of the oxygen of the atmospheres, and that conversely, if arte- 

 rial blood be submitted to regularly diminishing pressures of oxygen, at 

 first very little of the contained oxygen is given oif to the atmosphere, 

 then suddenly the gas escapes with great rapidity, and again disobeys 

 the law of pressures. 



Very little oxygen can be obtained from serum freed from blood- 

 corpu;des, even by the strongest mercurial air-pump, neither can serum 

 be made to absorb a large quantity of that gas; but the small quantity 

 which is so given up or so absorbed follows the laws of absorption 

 according to pressure. 



It must be, therefore, evident that the chief part of the oxygen is 

 contained in the corpuscles, and not in a state of simple solution. The 

 chief solid constituent of the colored corpuscles is hemoglobin, which 

 constitutes more than 90 per cent of their bulk. This body has a very 

 remarkable affinity for oxygen, absorbing it to a very definite extent 

 under favorable circumstances, and giving it up when subjected to the 

 action of reducing agents, or to a sufficiently low oxygen pressure. 

 From these facts it is inferred that the oxygen of the blood is combined 

 with hcemoglobin, and not simply dissolved; but inasmuch as it is com- 

 paratively easy to cause the haemoglobin to give up its oxygen, it is 

 believed that the oxygen is but loosely combined with the substance. 



Haemoglobin. Haemoglobin is a crystallizable body which consti- 

 tutes by far the largest portion of the colored corpuscles. It is inti- 

 mately distributed throughout their stroma, and must be dissolved out 

 before it will undergo crystallization. Its percentage composition is 



