ITS PHYSICAL, CHEMICAL, AND STRUCTURAL CHARACTERS. 157 



drawn, we have enumerated all the characteristics which can be made out by the 

 unassisted senses. 



138. When the Blood is examined with the Microscope, however, either im- 

 mediately upon being drawn, or whilst it is yet circulating in the vessels of the 

 living body (as in the foot of the Frog, the wing of the Bat, or any other mem- 

 branous expansion of similar transparency), it is seen that its apparent homo- 

 geneity is not real, but that it consists of two very different components. These 

 are, a transparent and perfectly colorless liquid, which is known as the Liquor 

 Sanguinis, and a set of Corpuscles, which are suspended in it : the great mass 

 of these last present a distinctly red hue, and it is to their presence alone that 

 the color of the blood is due ; but there are also to be seen, scattered among the 

 red, a few which are colorless, and which differ from the red in some other par- 

 ticulars presently to be noticed. On the other hand, when the Blood has been 

 drawn from the body, and is allowed to remain at rest, it undergoes a spontaneous 

 coagulation, in the course of which it separates into a red Crassamentum and a 

 nearly colorless Serum. The " crassamentum" or " clot" is composed of a net- 

 work of Fibrin ( 26, 27), in the meshes of which the Corpuscles, both red and 

 colorless, are involved, together with a certain amount of serous fluid. The 

 " serum/ ; which is the same with the "liquor sanguinis" deprived of its Fibrin, 

 coagulates by heat, and is, therefore, known to contain Albumen ; and if it be 

 exposed to a high temperature, sufficient to decompose the animal matter, a 

 considerable amount of earthy and alkaline Salts remains. Thus we have four 

 principal components in the Blood : namely, Fibrin, Albumen, Corpuscles, and 

 Saline matter. In the circulating blood, they are thus combined : 



Fibrin ~\ 



Albumen I In solution, forming Liquor Sanguinis. 



Salts J 



Corpuscles suspended in Liquor Sanguinis. 



But in coagulated blood, they are combined as follows : 

 P * , I Forming Crassamentum or Clot. 

 g , , I Remaining in solution, forming Serum. 



The change from the one condition to the other is due to the fibrillation of the 

 Fibrin, which usually takes place so speedily as to involve the Corpuscles float- 

 ing in the " liquor sanguinis" before they have time to subside, although, under 

 various conditions hereafter to be described (SECT. 3), it may occur in such a 

 manner that the clot, or a portion of it, is left colorless. The Fibrin, Albumen, 

 and Saline components of the Blood present no other characters than those which 

 have been already detailed in the general account of these substances (CHAP, n.) ; 

 and the only constituents remaining to be described, therefore, are the Cor- 

 puscular, which are not mere organic compounds, but have a regularly organized 

 structure. 



139. The Red Corpuscles of the Blood (commonly, but erroneously, termed 

 " globules") are cells of a flattened or discoidal form; which, in Man, as in most 

 of the Mammalia, have a distinctly circular outline. In the disks of Human 

 blood, when this is examined in its natural condition, the sides are somewhat 

 concave ; and there is a bright spot in the centre, which has been regarded by 

 many as indicating the existence of a nucleus ; though it is really nothing else 

 than an effect of refraction, and may be exchanged for a dark one by slightly alter- 

 ing the focus of the Microscope (Fig. 11). The form of the disk is very much 

 altered by various reagents; for the membrane which composes its exterior, or cell- 

 wall, is readily permeable by liquids ; so as to admit of their passage, according to 



