vi CIRCULATION OF BLOOD: ITS DISCOVERY 177 



gradually larger, while the intravascular portion still keeping 

 its round shape continuously diminishes in volume, till at last it 

 appears only as a mere shining point, and eventually disappears 

 altogether. The extra vasated leucocyte is then seen completely 

 free from the vessel ; it resumes its circular shape, and remains 

 motionless. 



The direct observation of diapedesis can be facilitated by staining 

 the leucocytes of the blood with methylene blue or other colouring 

 matters introduced into the dorsal lymph-sac of the frog the 

 method of Cohnheim. 



Recent researches have left no doubt that the diapedesis of 

 leucocytes is an active phenomenon, intimately connected with 

 their amoeboid mobility. The extravasation of red corpuscles (the 

 true haemorrliagia pc.r diapedesin, as divined by the ancients) is, on 

 the contrary, a passive process, depending either on rise of intra- 

 vascular pressure or on nutritional disturbances and lowered 

 resistance of the capillary walls. 



In the frog's peritoneum the extravasation of the erythrocytes 

 (from the capillaries rather than from the veins) is ' first noticed 

 after several hours, and becomes conspicuous only twenty-four 

 hours after the beginning of the experiment, in the capillary 

 network where the circulation is at a standstill and the block of 

 corpuscles is greatest. 



This observation led Cohnheim to the opinion that the leucocytes 

 penetrate through tiny pre-formed stomata in the vessel wall, by 

 which the erythrocytes can escape only when the openings are 

 abnormally enlarged by the active work of the leucocytes. The 

 opinion that prevails at present, however, is that no pre-formed' 

 stomata exist, and that the emigration proceeds by temporary, 

 openings, excavated by the pseudopodia of the leucocytes at the- 

 junction of the histologic il elements of the venous walls or capillary 

 endothelia. The erythrocytes, owing to the softness and elasticity 

 of their protoplasm, pass readily (perhaps even passively) through 

 the openings excavated by the leucocytes, as through a network. 



It is still doubtful if corpuscular extravasation (whether active, 

 as for the leucocytes, or passive, for the erythrocytes) is to be 

 regarded as a physiolog cal phenomenon, exaggerated under- 

 abnormal conditions of inflammatory irritation, or as an emphati- 

 cally pathological phenomenon. 



E. Hering adopted the former opinion, on the strength 

 of the following experiment. He injected a finely pulverised 

 aniline pigment into the blood of an animal, and after some time 

 examined the hepatic lymph, when he found numerous leucocytes 

 as well as erythrocyfces impregnated with pigment, but no free 

 granules of pigment in the lymph plasma. From this he con- 

 cluded that under normal conditions also certain leucocytes (and 

 possibly erythrocytes as well) migrate from the vascular system 



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