BLOOD. 105 



with respiration, or with the excretory organs, and perhaps also anything 

 which disturbs the healthy condition of the nervous system, will affect 

 the composition of the blood. Inflammatory diseases remarkably modify 

 its character. 



Blood, when first drawn, appears to be homogeneous ; but on cooling 

 it separates into two nearly equal parts, namely a fluid called Serum, and 

 a red solid portion called Crassamentum or clot. 



201. Serum. 



Serum is a thin yellowish or straw coloured fluid, somewhat thicker 

 than water, consisting principally of water with an admixture of albumen 

 and salts. 



During life serum, or a fluid very like it, is frequently separated from 

 the blood and effused, whenever the vessels become congested and over- 

 distended. 



202. Crassamentum. 



The Crassamentum or Clot is composed of fibrin holding the red cor- 

 puscles in its meshes, caught as they are sinking to the bottom. It owes 

 its colour to the red corpuscles. Fibrin, when deprived of them by wash- 

 ing in cold water, loses its colour and presents an appearance much 

 resembling bleached muscular fibre. Fibrin, when exposed to the air, 

 usually coagulates spontaneously, and it has the power of interlacing its 

 fibres and of contracting on itself. It is soluble in alkalies and acids, 

 but insoluble in water, oil, or spirits. It is a remarkable fact that fibrin 

 as such does not exist in the liquid blood during circulation, but is 

 formed during coagulation by the union of two substances, paraglobulin 

 and fibrinogen, with the aid of a ferment. 



203. The red corpuscles. 



The Red corpuscles are somewhat heavier than the other parts of the 

 blood. In shape they are like flattened discs, and often adhere by their 

 surfaces to each other and form piles like rouleaux of coins, especially in 

 inflammatory blood. Each corpuscle consists of a wall of tough elastic 

 membrane enclosing a substance called Hsematin, which is the red pig- 

 ment of blood. One of the component parts of hsematin is iron. 



The main function of the red corpuscles is to act as carriers of oxygen 

 to the tissues. The products generated in the tissues by oxidation are 

 returned in the venous blood in the form of carbonic acid gas, urea, 

 water, and other effete materials. During the oxidizing process caloric 

 is also set free. 



Many vessels in a state of health are too small to admit the red cor- 

 puscles ; but when their coats under the influence of inflammation are 

 distended, they do admit them. Hence a part hitherto devoid of colour 

 may become red. 



