CHAP. IT.] RESPIRATION. 457 



of haemoglobin with oxygen, while the change from arterial to 

 venous is, in part also, a reduction of oxyhaemoglobin : and the 

 difference of colour between venous and arterial blood depends 

 almost entirely on the fact that the reduced haemoglobin of the 

 former is of purple colour, while the oxyhaemoglobin of the lat- 

 ter is of a scarlet colour. 



There may be other causes of the change of colour, but these 

 are wholly subsidiary and unimportant. When a corpuscle 

 swells, its refractive power is diminished, and in consequence 

 the number of rays which pass into and are absorbed by it are 

 increased at the expense of those reflected from its surface ; 

 anything therefore which swells the corpuscles, such as the 

 addition of water, tends to darken blood, and anything, such as 

 a concentrated saline solution, which causes the corpuscles to 

 shrink, tends to brighten blood. Carbonic acid has apparently 

 some influence in swelling the corpuscles, and therefore may 

 aid in darkening the venous blood. 



280. We have spoken of the combination of haemoglobin 

 with oxygen as being a peculiar one. The peculiarity consists 

 in the facts that the oxygen may be associated and dissociated, 

 without any general disturbance of the molecule of haemoglobin, 

 and that dissociation may be brought about very readily. Hae- 

 moglobin combines in a wholly similar manner with other gases, 

 If carbonic oxide (monoxide) be passed through a solution of 

 haemoglobin, a change of colour takes place, a peculiar bluish 

 tinge making its appearance. At the same time the spectrum 

 is altered ; two bands are still visible, but on accurate measure- 

 ment it is seen that they are placed more towards the blue end 

 than are the otherwise similar bands of oxyhaemoglobin (see 

 Fig. 89, 6) ; their centres corresponding respectively to about 

 wave-lengths 572 and 533, while those of oxyhaemoglobin as 

 we have seen correspond to 578 and 539. When a known 

 quantity of carbonic oxide gas is sent through a haemoglobin 

 solution, it will be found on examination that a certain amount 

 of the gas has been retained, an equal volume of oxygen appear 

 ing in its place in the gas which issues from the solution. If 

 the solution so treated be crystallized, the crystals will have the 

 same characteristic colour, and give the same absorption spec- 

 trum as the solution ; when subjected to the action of the mer- 

 curial pump, they will give off a definite quantity of carbonic 

 oxide, 1 grm. of the crystals yielding 1*59 c.cm. of the gas. In 

 fact, haemoglobin combines loosely with carbonic oxide just as 

 it does with oxygen ; but its affinity with the former is greater 

 than with the latter. While carbonic oxide readily turns out 

 oxygen, oxygen cannot so readily turn out carbonic oxide. 

 Indeed, carbonic oxide has been used as a means of driving 

 out and measuring the quantity of oxygen present in any given 

 blood. This property of carbonic oxide explains its poisonous 



