68 PERSONAL APPEARANCES IN HEALTH AND DISEASE. 



puscles, and the difference in colour thus produced con- 

 stitutes the essential difference between the* blood which 

 flows/w;2 the heart in the arteries to all parts of the body, 

 and that which is returned to the heart from the tissues by 

 the veins. When ah artery is divided the blood that 

 jets forth with every beat of the heart is of a bright red 

 colour on the other hand, that which flows from a 

 divided vein in a constant stream is dark and purple. 

 The reason of this difference in colour between arterial 

 and venous blood arises from the fact that before entering 

 the left side of the heart to be sent through the body by 

 the arteries, the blood passes through the lungs, and then 

 the carbonic acid with which the venous blood is charged 

 is at once exchanged for oxygen by the process of 

 respiration. Again, the bright arterial blood carrying 

 oxygen to the tissues, the muscles, glands, &c., receives 

 carbonic acid in exchange for its oxygen, which goes to 

 maintain the tissues in their constant process of change, 

 and the dark carbon-laded blood courses .back to the 

 heart to be again renewed in the lungs. If from any 

 cause this process of oxygenation of the blood be carried 

 on imperfectly, or if the heart be unable to do its work 

 efficiently, so that the venous blood does not pass on 

 with sufficient rapidity, then the blood in the tissues is 

 less bright, and as will be seen later on, the surface of the 

 body denotes the darkening. 



