BLOOD. 337 



as in the case of individuals sleeping in a room where gas is escaping, is trace- 

 able directly to the carbon monoxide. Nitric oxide (NO) forms also with 

 hemoglobin a definite compound which is even more stable than the CO- 

 hemoglobin ; if, therefore, this gas were brought into contact with the blood, 

 it would cause death in the same way as the CO. 



Oxyhemoglobin, carbon-monoxide haemoglobin, and nitric-oxide haemoglo- 

 bin are similar compounds. Each is formed, apparently, by a definite combina- 

 tion of the gas with the haemochromogen portion of the haemoglobin molecule, 

 and a given weight of hemoglobin unites presumably with an equal volume of 

 each gas. In marked contrast to these facts, Bohr l has shown that haemoglobin 

 forms a compound with carbon-dioxide gas, carbo- haemoglobin, in which the 

 quantitative relationship of the gas to the haemoglobin differs from that shown 

 by oxygen. In a mixture of O and CO 2 each gas is absorbed by haemoglobin 

 solutions independently of the other, so that a solution of haemoglobin nearly 

 saturated with oxygen can unite with as much CO 2 as though it held no oxygen 

 in combination. Bohr suggests, therefore, that the O and the CO 2 must unite 

 with different portions of the hemoglobin the oxygen with the pigment portion, 

 the haemochromogen, and the CO 2 possibly with the proteid portion. It seems 

 probable that haemoglobin plays a part in the transportion of the carbon 

 dioxide as well as the oxygen of the blood, but its exact value in this respect 

 as compared with the blood-plasma, which also acts as a carrier of CO 3 , has 

 not been definitely determined (see Respiration). 



Presence of Iron in the Molecule. It is probable that iron is quite 

 generally present in the animal tissues in connection with nuclein compounds, 

 but its existence in haemoglobin is noteworthy because it has long been known 

 and because the important property of combining with oxygen seems to be 

 connected with the presence of this element. According to the analyses 

 made, the proportion of iron in hemoglobin varies somewhat in different 

 animals : the figures given are from 0.335 to 0.47 per cent. The amount of 

 hemoglobin in blood may be determined, therefore, by making a quantitative 

 determination of the iron. The amount of oxygen with which hemoglobin 

 will combine may be expressed by saying that one molecule of oxygen will 

 be fixed for each atom of iron in the hemoglobin molecule. In the decom- 

 position of hemoglobin into globulin and hematin or globulin and hemo- 

 chromogen, which has been spoken of above, the iron is retained in the 

 h?ematin. 



Crystals. Hemoglobin may be obtained readily in the form of crystals 

 (Fig. 86). As usually prepared, these crystals are really oxyhemoglobin, but 

 it has been shown that reduced hemoglobin also crystallizes, although with 

 more difficulty. Hemoglobin from the blood of different animals varies to a 

 marked degree in respect to the power of crystallization. From the blood of 

 the rat, dog, cat, guinea-pig, and horse, crystals are readily obtained, while 

 hemoglobin from the blood of man and of most of the vertebrates crystallizes 

 much less easily. Methods for preparing and purifying these crystals will be 

 1 Skandivavisches Archivfur Physiologic, 1892, Bd. 3, S. 47. 



