DISEASES OF CIRCULATORY SYSTEM. 



Before entering upon a consideration of the diseases which affect 

 the different parts that go to make up the circulatory system, by 

 means of and through which the blood traverses the body, an at- 

 tempt must be made to explain of what these parts consist and 

 how the function of the circulation of the blood is maintained and 

 carried on. In order to keep up that regular and constant flow of 

 blood through all the intricate channels of the animal frame that 

 is necessary for the maintenance of life and health, it will be ap- 

 parent that some force or driving power is an essential requi.site;^ 

 without such force it would be no more possible that the body of 

 fluid, which is represented in the animal body b}' blood, could be 

 kept constantly and regularly flowing than that a number of rail- 

 way carriages can be kept moving without the power generated in 

 the engine by means of steam; the weight of the fluid demands a 

 propelling power to keep the same in motion. The engine which 

 keeps the blood flowing is the heart; this organ is situate in the 

 cavity where the lungs are located, and its position is intermediate 

 between the two lungs, rather nearer the front than the back of 

 the chest; the heart is covered externally by a very fine mem- 

 brane called the pericardium, which has a smooth surface out- 

 wardly, so that it may glide easily against the outer surface of the 

 lungs, already described as the pleura. The heart is divided into 

 four compartments or chambers, two above, called auricles, and 

 two below, called ventricles; an auricle over a ventricle on the 

 right side, which communicate with one another by orifices that can 

 be closed by means of valves, and the same on the left side of the 

 heart. In connection with the heart there are sets of tubes which 

 convey the blood away; the first is that which passes the blood 

 from the right ventricle through the capillaries of the lungs that 

 the blood may come in contact with the fresh-air passage in and 

 out as the animal breathes, in order that it may be purified; it 

 then returns through the pulmonary veins back into the left 



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