584 Anatomy, Physiology, and Pathology of the 



shows an artery, a, terminating in a capillary rete of vessels, and 

 a vein, b, arising therefrom ; it will also be observed that, in 

 accordance with the facts we have described, the former is repre- 

 sented light, and the latter dark in colour. 



The brightness of the arterial blood is due to the presence of 

 the oxygen of the air acting on its coloured corpuscules, and which 

 it receives by its passage throrgh the lungs; and the darkness 

 of the venous blood, to the influence of the carbonic acid of the 

 system on the same bodies. These gases, however, affect only 

 the pigment of the red corpuscules to produce this altered appear- 

 ance of the circulating fluid ; and consequently the corpuscules 

 can only be viewed as the indirect carriers of the oxygen into, 

 and the carbonic acid out of the body. 



I have before stated that the various tissues of the frame are 

 undergoing a continual waste or change, and therefore they need a 

 constant reparation, which is provided for by the appropriation of 

 the blood by the capillaries. The thinness of the walls of these 

 vessels allows of a transudation of the liquid fibrine, which being 

 imbibed by the surrounding structures administers to their sup- 

 port ; while any excess is carried back into the circulation by the 

 lymphatic absorbents. The metamorphosis, however, of the tis- 

 sues furnishes both carbon and hydrogen, and with these the oxygen, 

 which has been conveyed into the capillaries by the red corpus- 

 cules, unites, forming thereby carbonic acid and watery vapour. In 

 this process heat is evolved ; and as it takes place in every part of 

 the system, so it follows that the temperature of the body is every- 

 where kept up to its standard, viz., about 99° of Fahrenheit, inde- 

 pendent of external causes. An animal may thus be said to carry 

 with him a self-supplying furnace, which continues in active 

 operation so long as health and vigour of constitution remain. It is 

 this generation of heat by chemical union which is designated 

 animal heat. By the loss of some of its fibrine, and by the pre- 

 sence of carbonic acid, the blood is now rendered unfit for the 

 purposes of life, and in this condition it returns to the heart by 

 the veins (see fig. 6). Near to this organ it receives a fresh sup- 

 ply of nutritive matter through the medium of the thoracic duct 

 (see fig. 2), and passing from the heart to the lungs it again obtains 

 the required oxygen, and parts with the carbonic acid and watery 

 vapour (see fig. 7). 



The function thus performed by the lungs, of which we must 

 speak more at length, will be better understood by again referring 

 to the diagram, fig. 7. One portion of this diagram represents the 

 four cavities of the heart laid open, and the vessels which are 

 going to and from them ; and the other a branch of the windpipe 

 terminating in the air-cells of the lungs, which are surrounded by 

 a network of capillaries indicating the change in the colour of the 

 blood. The intervening arrows show the course of the blood to 



