186 THE BODY AT WORK 



If the hand be held between a spectroscope and the source 

 of light, in such a position that the beam passes through the 

 thin tissue of two fingers where they are in contact, the spec- 

 trum of oxyhsemoglobin is obtained. If now the circulation 

 through the fingers be impeded by putting strong indiarubber 

 bands round them, the blood becomes venous, and the two 

 narrow bands of oxyhaemoglobin give place to the broad band 

 of reduced haemoglobin. 



Although very soluble, haemoglobin may be obtained in 

 crystals, the form of which varies in different animals. When 

 obtained from human blood, the crystals are rhombic prisms ; 

 from the guinea-pig, tetrahedra ; from the squirrel, hexagonal 

 plates. Yet it is unlikely that the haemoglobin of one 

 animal differs chemically from that of another in any proper 

 sense of the term. Probably the form of the crystals depends 

 upon the amount of water of crystallization. The apparent 

 polymorphism of haemoglobin may be associated with the great 

 size of its molecules (cf. p. 66). 



Even when in the crystalline form, haemoglobin can take up 

 oxygen ; but the difficulties which attend its purification and 

 crystallization render somewhat uncertain the amount of 

 oxygen which a gramme of crystallized haemoglobin can absorb. 

 In solutior, 1 gramme can take up 1-34 cubic centimetres. 

 The whole of the haemoglobin of the body would, therefore, if 

 it were all in the oxidized condition, hold about 4 grammes of 

 oxygen. 



It is not with oxygen alone that haemoglobin can combine. 

 It can absorb the same volume of carbonic oxide or of nitric 

 oxide gas. Both of these gases it holds more firmly than 

 oxygen. Neither carbonic oxide-haemoglobin nor nitric oxide- 

 haemoglobin is of any use to the tissues. If the blood becomes 

 charged with the fumes of carbonic oxide (CO) given off by a 

 coke- fire, this gas proves extremely poisonous. The blood does 

 not lose it in its circuit through the body, nor is it exchanged 

 for oxygen in the lungs. 



The instability of the compound of haemoglobin and oxygen 

 is shown under the air-pump. The pressure of air in the open 

 equals 760 millimetres of mercury. When the pressure falls to 

 about 250 millimetres, the oxygen is rapidly given off. This is 

 a matter of considerable interest in its bearing upon the ques- 



