THE BLOOD. 157 



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 off 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 plasma freed from blood 

 corpuscles, even by the strongest mercurial air-pump, neither can it 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 ac- 

 cording 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 Jicemoglobin, 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 

 Jicemoglobin, and not simply dissolved; but inasmuch as it is compara- 

 tively 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 intimately 

 distributed throughout their stroma, and must be dissolved out before 

 it will undergo crystallization. Its percentage composition is C. 53.85; 

 H. 7.32; N. 16.17; 0. 21.84; S. .63; Fe. .42. Jacqnet gives the 

 empirical formula for the haemoglobin of the dog, C 758 H 1203 N ]95 S 3 Fe 

 216 . The most interesting of the properties of haemoglobin are 

 its powers of crystallizing and its attraction for oxygen and other 

 gases. 



Crystals. The haemoglobin of the blood of various animals possesses 

 the power of crystallizing to very different extents (haemoglobin). In 

 some animals the formation of crystals is almost spontaneous, whereas 

 in others it takes place either with great difficulty or not at all. Among 

 the animals whose blood coloring-matter crystallizes most readily are 

 the guinea-pig, rat, squirrel, and dog; and in these cases to obtain 

 crystals it is generally sufficient to dilute a drop of recently-drawn blood 

 with water and to expose it for a few minutes to the air. Light seems 

 to favor the formation of the crystals. In many instances other means 



