622 THE INTERNAL SECRETIONS AND IMMUNITY. 



into protective activity is, indeed, suggestive when their ex- 

 tensive distribution is recalled. As is well known, endothelial 

 cells line all spaces, channels, or cavities, large or small, that 

 permeate the great connective-tissue system. The peritoneal, 

 pleural, pericafdial, and arachnoid cavities, the lymph-vessels, 

 and the inner surface of blood-vessels are thus protected 

 throughout with a complete layer of these elements. 



Especially important is it to recall, in this connection, a 

 few classic features, namely: that the capillary system is prac- 

 tically made up of endothelial cells joined edge to edge; that 

 the circulation of the blood in these minute vessels is relatively 

 slow; that the peripheral portion of the blood coursing through 

 them, that part in contact with their endothelial walls, adheres, so 

 to say, to the latter and is further delayed; and, finally, that 

 serous membranes are, like the capillaries, endothelial struct- 

 ures. The peritoneum, the pleura, the pericardium, the me- 

 ninges, etc., thus become an extension of the capillary endothelial 

 system, and when, in the light of the doctrines advanced in this 

 work, these serous structures and the capillaries are consid- 

 ered as one, we have constituted an extensive system -of epura- 

 tion, the physiological mechanism of which seems to us to be 

 represented in the following summary: 



The "blood-vessels, supplied with a muscular coat, when un- 

 duly contracted through the excessive functional activity of the 

 adrenal system induced by toxics, force the blood into the peripheral 

 capillaries, and, in some disorders, into those of serous membranes; 

 all these capillaries, owing to their phagocytic endothelial walls 

 and the wandering phagocytic leucocytes of the blood-plasma, thus 

 become collectively transformed into a vast immunizing field. 



It may prove interesting, in this connection, to refer to 

 the importance of the capillary system as viewed by Claude 

 Bernard 15 when compared to the balance of the vascular sys- 

 tem: "The larger vessels, the arteries, the veins," writes this 

 great physiologist, "are but streets which enable us to meander 

 through a city; but with the capillaries we can enter the houses, 

 in which we can directly study the modes of life, occupations, 

 and customs of the inhabitants. Thus, when a toxic or medic- 



18 Claude Bernard: "Physiologic Operatoire," 1879; quoted by M. Duval, 

 loc. cit. 



