436 HEALTH AND DISEASE 



that seen in the white of egg before and after boiling, or in gelatine before 

 and after setting. The consistence of the clot is about equal to that of red- 

 currant jelly. After the lapse of a few minutes a further change may be 

 seen. The clot contracts, and minute drops of a clear fluid begin to exude 

 from the surface. This is the separation of the serum from the clot; and, 

 as the contraction continues for many hours, the clot is ultimately covered 

 and surrounded by a layer of clear fluid of considerable depth. When the 

 separation of the freshly-drawn blood into three layers has taken place, the 

 contraction of the upper layer of plasma, being unhindered by the presence 

 of corpuscles, causes the upper surface to be much depressed in the centre, 

 and its colour being yellowish, such clot is spoken of as being " bufl"ed and 

 cupped ". It is, of course, not observed when clotting has taken place too 

 quickly for the corpuscles to sink through the plasma. Coagulation of the 

 blood is believed to result from the breaking up into two parts of a proteid 

 substance named fibrinogen, naturally existing in solution in the plasma. 

 One of these parts is a globulin which, under the influence of a ferment 

 existing in the white corpuscles, remains in solution; the other is fibrin, 

 which immediately solidifies, forming a delicate net-work in the meshes of 

 which the corpuscles are entangled, and which, subsequently contracting, 

 squeezes out the serum. The calcium salts also play an important part in 

 the process. 



THE MECHANISM OF THE CIRCULATION 



The blood, the characters of which have just been considered, circulates 

 through the body by the agency of the heart and blood-vessels. The 

 heart is a portion of the vascular system, consisting of a compact but 

 hollow mass of muscle that acts rhythmically as a pump, and, owing to 

 the presence of valves suitably placed, drives the blood in one constant 

 direction through the body. It is placed in the thorax or chest, and is 

 protected from injury by the breast- bone and the firm but elastic ribs 

 and spine, and also owing to its being enveloped to a large extent by 

 the spongy tissue of the lungs. It is enclosed in a tough membranous 

 bag named the j^ericardium, lined internally by a serous membrane, the 

 smooth and polished surfaces of which, moistened with the fluid they 

 secrete, reduce friction to a minimum. The general course pursued by 

 the blood is from the heart, through the arteries and their ramifications, 

 named capillaries, onward to the veins, by which it is again conducted 

 to the heart. Although in appearance single, the heart is really a double 

 organ, the two parts being united for the sake of economy of space and 

 material, and also to enable them to work equably and simultaneously. 



