188 LECTURE VII. 



pus-corpuscles, but still more so in pathological condi- 

 tions when the blood or other parts are full of these ele- 

 ments. You can imagine how apt the question is to 

 present itself, which has already been seriously raised by 

 Addison and Zimmermann, whether pus-corpuscles are 

 not merely extravasated colourless blood-cells, or vice 

 versa, whether the colourless blood-cells found within 

 the vessels are not pus-corpuscles which have been ad- 

 mitted into them from the exterior. We are here called 

 upon for the first time to make the practical application 

 of the principles which I laid down with regard to the 

 specific nature and heterology of elements (p. 92.) A 

 pus-corpuscle can be distinguished from a colourless 

 blood-cell by nothing else than its mode of origin. If 

 you do not know whence it has come, you cannot say 

 what it is ; you may conceive the greatest doubt as to 

 whether you are to regard a body of the kind as a pus- 

 or a colourless blood-corpuscle. In every case of the 

 sort the points to be considered are, where the body 

 belongs to, and where its home is. If this prove to be 

 external to the blood, you may safely conclude that it is 

 pus ; but if this is not the case, you have to do with 

 blood-cells. 



