STRUCTURAL CHANGES DUE TO DAMAGE. 301 



eytes. This is particularly the case when the cause of the inflam- 

 mation is an infection with bacteria. Under those circumstances 

 the leucocytes that emigrate from the blood accumulate in great 

 numbers in the tissues around the site of infection. 



The leucocytes, by their passage through the cement between the 

 endothelia, open minute channels through which the red corpuscles 

 of the blood may be pressed into the surrounding tissues, when they 

 come in contact with the vascular wall after stasis (complete arrest 

 of the circulation) has become established. These corpuscles are 

 soft, and can be forced through orifices much smaller than their 

 normal diameters; but the number that escape from the vessels 

 varies greatly in different cases of inflammation, and it is probable 

 that the integrity of the vascular wall is more affected when the 

 number is great than when it is slight, and that the leucocytes 

 prepare the way for only a portion of the red corpuscles that escape 

 from the vessel in those cases in which large numbers pass into the 

 surrounding tissues. The escape of red corpuscles from a vessel 

 without obvious rupture of its walls is called " diapedesis." 



As a result of the processes already described, it will be observed 

 that three of its constituents pass from the blood into the sur- 

 rounding tissues : (1) serum, (2) leucocytes, and (3) red blood-cor- 

 puscles. These constitute what is known as the u exudate." But 

 to these three a fourth constituent is soon added, namely, fibrin. 

 The formation of fibrin is still awaiting a perfectly clear explana- 

 tion, but it is usually assumed to be the result of the interaction of 

 three substances : (1) fibrinogen, derived from the plasma of the 

 blood ; (2) fibrinoplastin and (3) fibrin-ferment, both of which may 

 come from the bodies of cells. In the exudate of acute inflamma- 

 tion all of these elements necessary for the formation of fibrin are 

 present in greater or less amount. (See explanation of fibrin- 

 formation on p. 127.) As found in the tissues, therefore, the exu- 

 date consists of serum, fibrin, leucocytes, and red corpuscles (Fig. 

 268). But in different cases their relative abundance differs, and 

 the acute inflammations have been roughly classified according to 

 the character of the exudate. Thus, the serous inflammations are 

 those in which serum predominates in the exudate. In like 

 manner inflammations are designated by the terms fibrinous, 

 hsemorrhagic, and purulent (when the leucocytes predominate), or 

 sero-fibrinous, sero-purulent, fibrino-purulent, etc. These terms 

 are descriptive, and merely indicate variations in the proportions 



