DISEASES OF THE CIRCULATORY SYSTEM. 1 33 



auricle of the heart, and from this into the left ventricle, directly 

 under it, in connection with which is the largest blood tube of the 

 bod}-, called the aorta; from this large tube branches, called 

 arteries, divide and subdivide to the various parts and organs of 

 the body; these arteries or branches of the aorta in the course of 

 their distribution over the system become so small and fine that 

 the name capillaries has been given to them ; in the course of its 

 passage through these capillaries the blood undergoes considerable 

 change, as ma}' be noted from the alteration in color; from a vivid 

 bright red it becomes dark or blue red, and having thus done its 

 work it becomes necessary to return it to the heart to be repurified, 

 and so it passes from the capilliaries into the veins, which in turn 

 become gradually larger and larger, until the w^hole stream is col- 

 lected into two large tubes which empty themselves again into the 

 right auricle of the heart, and here the round of circulation begins 

 once more. The heart itself consists of muscular walls of varying 

 thickness, and by means of the contractile powers of these muscu- 

 lar walls, which are stimulated to action by the nervous system, 

 the blood is propelled into the arteries, thence through the capil- 

 laries and back again through the veins; each time the heart con- 

 tracts a certain quantity of blood is driven forcibly into the aorta, 

 and that which is expelled at one contraction forces further on 

 that quantity which preceded it, and so on as each contraction ful- 

 fils its work, until continuous pressure on the whole stream is kept 

 up from behind, and so the circulation is maintained; it is con- 

 sidered that the walls of the arteries by means of their elastic 

 recoil contribute some force to aid the heart in the performance of 

 its function; the arteries are open tubes with no internal obstacles 

 to the flow of blood, and their w^alls consist of muscular and 

 elastic fibres which expand as the heart contracts and forces blood 

 into them; while during the pause which takes place when the 

 heart is momentarily at rest, in consequence of there being no 

 pressure, recoil of the elastic fibres takes place and in this way 

 the blood is kept in motion. The veins assist the circulation 

 through the medium of cup-shaped valves, which are attached to 

 their inner walls; the object of these being to prevent the blood 

 from receding or moving backwards; during the period of con- 

 traction, when the blood is being forced on in its proper direction, 

 these cup-shaped valves lie flat against the wall of the veins, and 



