CHAPTER VIII. 

 THE VASCULAR MECHANISM. 



SECTION I. 



THE life of the muscles, of the nervous system, and of every tissue of 

 the body depends upon their receiving an adequate supply of food and 

 oxygen ; an.d one of the most important functions of the blood is to 

 convey oxygen and nutritive material to the tissues and to carry away 

 carbon dioxide and waste products which are formed by the tissues. 

 In order that this function may be carried out, the heart and blood- 

 vessels furnish the mechanism by which a constant circulation of the 

 blood throughout the body is maintained ; and by means of the central 

 nervous system the activities of this mechanism can he varied in 

 response to the ever-changing needs, either of the body as a whole, or 

 of its different parts. 



The heart acts as a pump, and drives the blood along the arteries, 

 through the capillaries and veins and back to the heart. The actual 

 interchange of nutritive material and waste products between the 

 blood and tissues takes place solely through the walls of the capillaries, 

 and the entire circulatory mechanism is adapted to maintain the con- 

 ditions most favourable to this interchange. 



THE HEART AND BLOOD VESSELS, 

 t 



The heart is a hollow muscular organ lying in the thorax between 

 the lungs, and slightly to the left of the middle line of the body. It is 

 conical in shape, the apex being directed downwards, and is divided by 

 a septum into right and left halves, which do not communicate directly 

 with each other. Each half consists of two chambers, an upper thin- 

 walled auricle (atrium) and a lower thick-walled ventricle. Into the 

 right auricle open the superior vena cava, bringing blood from the head 

 and upper limbs, the inferior vena cava, conveying blood from the rest 

 of the body, and the coronary sinus. The right auricle opens into the 

 right ventricle by an orifice guarded by a valve with three triangular 



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