BLOOD AND LYMPH. 8/ 



certain of the mammalia, are so considerable that 

 they can be distinguished with certainty from one 

 another by microscopic measurements. It should, 

 on the other hand, never be forgotten that the red 

 blood-cells of certain other mammalia, e. g., the dog, 

 have so nearly the same average diameter as those 

 of man that they cannot be distinguished with ab- 

 solute certainty by measurements. In other verte- 

 brates the form and character of the red blood-cells 

 differ greatly from those above described ; being for 

 the most part oval, and having a distinct nucleus. 



The intercellular substance or plasma of freshly 

 drawn blood is perfectly homogeneous ; but if we 

 allow it to coagulate, we find, on subjecting it to 

 microscopical examination, that in addition to the 

 above-described elements, a multitude of delicate 

 filaments lie among the cells, stretching in all direc- 

 tions, and joining each other at frequent intervals, 

 forming an irregular-meshed net. We find, more- 

 over, if we examine a clot from which the red cells 

 have been carefully removed or rendered invisible, 

 that in many places the filaments are grouped around 

 irregular-shaped granules, looking like those above 

 described in normal blood ; and if the process of 

 coagulation be carefully observed, they may be seen 

 to actually shoot out from these granules, which 

 thus seem to form starting-points for their forma- 

 tion. The substance which thus separates from the 

 plasma is called fibrin. 



