PERIPHERIC ORGANS OF THE CIRCULATION. 161 



however, so simple as an examination of them, without the 

 use of reagents, would lead us to suppose : the researches of 

 Auerbach, Eberth, and Aeby, and the method of impregna- 

 tion by nitrate of silver, employed by Chrzonsczewsky, have 

 proved that the capillaries are lined with a pavement epi- 

 thelium (endothelium) exactly similar to that which forms 

 the inner layer of the arteries and the veins: outside this 

 endothelium the capillary coat is formed by a layer of cells, 

 placed close together ; so that we can no longer consider the 

 capillaries as produced by the end to end fusion of cells, whose 

 cavity would thus become the lumen (or interior space), and 

 the membranes the coat of the capillary. This way of 

 regarding the development of the capillaries was first sug- 

 gested by Schwann and Kolliker, as the result of their 

 experiments on the tail of young toads, and appeared to con- 

 firm the experiments of Balbiani on the cicatrization of 

 wounds in the same animals ; this theory, however, could 

 not stand before the discovery of an endothelium in the 

 cavity of the capillary ; from that time, this cavity has no 

 longer been looked upon as an intracellular, but as an inter- 

 cellular, space. The study of its development (His, Afana- 

 sieff, Rouget) proves that the capillaries not only possess this 

 endothelium, but that it is enclosed in another layer of cells : 

 the coat of the capillaries, when in course of development, is 

 composed on either side of two layers of cells placed end to 

 end, in such a manner that the vascular cavity is a canal 

 hollowed out between these double layers of cells. 1 



1 The structure of the capillaries and the small vessels (small 

 arteries and veins) does not as yet enable us to explain the phe- 

 nomenon of diapedesis, or protrusion of the globules, which some 

 observers have witnessed, and which many pathologists look upon 

 as one of the sources of suppuration. We have seen that the 

 white globules of the blood and the globules of the pus are exactly 

 similar, as also the globules of the lymph, whence the theory arose 

 that the pus globules were only white globules of the blood which had 

 left the vessels. Cohnheim (1869) asserts that, in his investigations 

 on inflammation of the cornea and the mesentery of the frog, this 

 hypothesis was verified by experiment, and that he saw the diapedesis 

 of the white globules. Hayem has made the same observations, and 

 states, moreover, that diapedesis of the red globules also takes place, 

 especially under the influence of an excess of pressure produced by 

 ligature of the veins. This question of pathological physiology is 

 too important tp be passed over here, but it is at the same time one 

 on which there is so much difference of opinion that we can only 

 mention it. The theory of diapedesis has many partisans in the 



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