PERSONAL APPEARANCES IN HEALTH AND DISEASE. 67 



only just tinged by reason of the entanglement of a few 

 of the red corpuscles, which have not had time to sink to 

 the bottom of .the vessel before the clot set. A micro- 

 scopical examination of a portion of the lower part of 

 the clot will show that there the fibrine has entangled 

 nothing but red corpuscles, whilst hardly any of these 

 corpuscles will be found in the upper strata. Again it 

 has been found that pale blood, or the blood of pale 

 persons, contains fewer red corpuscles than the average 

 quantity in health, and even quite recently it has been 

 ascertained that the red corpuscles themselves may contain 

 less colouring matter than the normal. The physiologist 

 has long since succeeded in separating this colouring 

 matter in bulk, and obtaining it in the crystalline state. 



The chief constituent of the blood pigment or hcemo- 

 globin, is carbon, which forms more than one-half of it ; 

 oxygen, nearly a quarter; nitrogen and hydrogen make 

 up all but a small fraction of the remainder, which 

 yields sulphur and iron. The most important property it 

 possesses is the power of combining with oxygen, and 

 when this combination occurs the colour of the blood 

 changes ; it becomes brighter and more scarlet. If, how- 

 ever, the oxidised blood be exposed to carbonic oxide or 

 carbonic acid, the colour changes to dark purple. This 

 readiness to combine with oxygen and to give it up again 

 appears indeed to be the essential property of the red cor- 



