194 THE CAT. [CHAP. vn. 



but soon drops of clear fluid exude from it, and collect to form what 

 is called the serum, while the solid mass left behind is the clot. 



Sometimes the clot is of a lighter colour above than below, show- 

 ing that the clot itself consists of two elements, as is indeed the case. 

 The clear part of it, is that substance which it has been said may be 

 obtained by whipping, namely, " fibrin ; " while the red part, when 

 examined by the microscope, is found to owe its colour to the presence 

 of an immense multitude of minute coloured discoidal bodies called 

 corpuscles* and which may be seen to float freely about in quite 

 fresh blood, or in blood which (from the addition of salt or some 

 other suitable matter, or from being kept at a low temperature) is 

 prevented from coagulating. 



Thus fresh blood is found to consist of corpuscles floating in a 

 fluid the liquor sanymms which fluid yields both fibrin and serum. 

 When coagulation takes place the fluid of the blood separates into 

 two parts. One, the fibrin, solidifies, and, by entangling the cor- 

 puscles amongst its filaments, forms the clot, while the remaining 

 part of the liquor sanguinis escapes as the serum. 



The coloured, or RED CORPUSCLES are disc-like structures, only about 

 the j*Vo of an inch in diameter, or even less. They are circular in 

 outline, and each flattened side is concave and medianly depressed, 

 so that each disc is thinnest from side to side in its middle, with a 

 somewhat enlarged circumference. 



These corpuscles have no limiting membrane, though their ex- 

 terior is somewhat denser than their interior, and they are soft and 

 elastic. They exhibit no interior structure and no nucleus, but they 

 consist of haemoglobin containing iron. Each is of a yellowish red 

 colour, but by their excessive multitude they produce the deep red 

 colour of the blood. In blood drawn from the body they tend to run 

 together in strings, applying themselves face to face like piles of 

 coins. 



Besides the red corpuscles, the blood also contains a variable 

 quantity of WHITE CORPUSCLES. These are much less numerous than 

 are the red, there being only some two or three white to a thousand 

 of the red, though the proportion increases after eating. They are 

 also rather larger than are the red corpuscles ; but their distinguish- 

 ing characteristics (besides colourlessness) are their possession of 

 a nucleus in their granular contents, and their spheroidal or irregular 

 form. The white corpuscles, in fact, have the power of spontaneously, 

 so to speak, altering their shape by protruding portions of their 

 substance in an irregular manner and in all directions. This change 

 of form is however effected very slowly, so that careful observation 

 for several minutes, or several observations at intervals of about a 

 minute, are needed to detect it. These movements are sometimes 

 termed amoeboid, from their resemblance to the movements exhibited 

 by some of the lowest animals. 



The blood, while within the body, during life, is really A TISSUE, 



* Which, make up about a third of tlio volume of the mass of the blood. 



