134 NORMAL HISTOLOGY. 



in the production of fibrin, perhaps by the liberation of fibrin- 

 ferment. 



Minute globules of fat are occasionally present in the blood, 

 especially during digestion. 



The lymph, like the blood, consists of a fluid portion, the plasma, 

 and corpuscles held in suspension. 



The plasma, as would be anticipated from its origin, is very 

 similar in composition to that of the blood. 



The corpuscles are, for the most part, identical with the small 

 leucocytes (lymphocytes) of the blood, which derives its supply of 

 those cells from the lymph flowing into it. 



The chyle is the lymph found in the lacteal lymphatics during 

 digestion. When absorption of the products of digestion is in 

 progress this lymph contains a great number of globules of fat, 

 some so minute as to be barely visible under the microscope. In 

 the intervals between absorption this lymph does not differ from 

 that found in the other lymphatics of the body. 



Fibrin may present the appearance of a delicate network of 

 extremely fine fibres, somewhat resembling a cobweb, or these 

 fibrils may be aggregated into larger threads variously interwoven, 

 or they may be still further condensed to form masses of a hyaline 

 character. The fibres may undergo a disintegration into granules, 

 when their fibrinous nature is not readily revealed. Fibrin is not 

 found in the body under normal conditions, but separates from the 

 blood if the circulation be arrested for any considerable length of 

 time. It appears to be the result of the interaction of four sub- 

 stances : fibrinogen, fibrinoplastin, fibrin-ferment, and salts of lime. 

 The latter are always present in the tissues ; fibrinogen exists in 

 the plasma of the blood and lymph, and is, therefore, very widely 

 distributed. The fibrinoplastin is believed to be derived from the 

 bodies of cells that have undergone some destructive change; and 

 the ferment may be derived from the same source. These four 

 substances are present when the flow of blood through the ves- 

 sels has been seriously checked for a considerable period ; fibrin is 

 then formed, causing a coagulation of the blood. Such a clot, 

 within a vessel during life, is called a "thrombus." Coagulation 

 takes place more rapidly if there be a destruction of tissue; e. 7., 

 a break in the wall of the vessel. It may also be occasioned by a 

 roughness on the internal surface of the vessel, if the flow of blood 

 over that obstruction is seriously retarded. In such a case the 



