340 COLOUR OF VENOUS AND ARTERIAL BLOOD. [BOOK 11. 



arterial blood depends almost entirely on the fact that the reduced 

 haemoglobin of the former is of purple colour, while the oxy- 

 hsemoglobin of the latter 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. 



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. Haemoglobin 

 combines in a wholly similar manner with other gases. If car- 

 bonic oxide be passed through a solution of haemoglobin, a change 

 of colour takes place, a peculiar bluish tinge making its ap- 

 pearance. At the same time the spectrum is altered ; two bands 

 are still visible, but on accurate measurement it is seen that they 

 are placed more towards the blue end than are the otherwise 

 similar bands of oxyhaemoglobin (see Fig. 58. 6) ; their centres corre- 

 sponding 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 appearing 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 

 spectrum as the solution; when subjected to the action of the 

 mercurial 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 nature. 

 When the gas is breathed, the reduced and the unreduced haemo- 

 globin of the venous blood unite with the carbonic oxide, and 

 hence the peculiar bright cherry-red colour observable in the blood 

 and tissues in cases of poisoning by this gas. The carbonic oxide 

 .haemoglobin, however, is of no use in respiration ; it is not an 



