38 



BLOOD. 



What we do know is that in blood soon after it has been shed the body 

 which we have called fibrinogen is present, as also the body which we have 

 called fibrin ferment, that the latter acting on the former will produce 

 fibrin, and that the appearance of fibrin is undoubtedly the cause of what 

 is called clotting. We seem justified in concluding that the clotting of shed 

 blood is due to the conversion by ferment of fibrinogen into fibrin. The 

 further inference that clotting within the body is the same thing as clotting 

 outside the body, and similarly due to the transformation of fibrinogen by 

 ferment into fibrin, though probable, is not proved. We do not yet know 

 the exact nature and condition of the blood within the living bloodvessels, 

 and until we know that we cannot satisfactorily explain why blood in the 

 living bloodvessels is usually fluid but can at times clot. 



THE CORPUSCLES OF THE BLOOD. 



The Red Corpuscles. 



24. The redness of blood is due exclusively to the red corpuscles. 

 The plasma as seen in thin layers within the living bloodvessels appears 

 colorless, as does also a thin layer of serum ; but a thick layer of serum 

 (and probably of plasma) has a faint yellowish tinge due, as we have 

 said, to the presence of a small quantity of a special pigment. 



The corpuscles appear under the microscope as fairly homogeneous, im- 

 perfectly translucent biconcave discs with a diameter of 7 to 8 (i. and a 



[FIG. 5. 



FIG. 6. 



FIG. 5. Human Blood as seen on the Warm Stage (magnified about 1200 diameters) : r, r, 

 single red corpuscles seen lying flat; r', r', red corpuscles on their edge and viewed in profile ; r", 

 red corpuscles arranged in rouleaux ; c, c, crenate red corpuscles; p, a finely granular pale cor- 

 puscle; <7, a coarsely granular pale corpuscle. Both have two or three distinct vacuoles, and 

 were undergoing changes of shape at the moment of observation ; in g a nucleus also was visible. 



FIG. 6. Human Red Corpuscles Lying Singly and Collected into Rolls. (As seen under an 

 ordinary high power of the microscope.)] 



thickness of 1 to 2 //. Being discs they are circular in outline when seen 

 on the flat, but rod-shaped when seen in profile. [Fig. 5.] Being bicon- 

 cave, with a thicker rounded rim surrounding a thinner centre, the rays of 



