CHAP, xiii.} CIRCULATION. 213 



to the exchanges of fluids and of gases. Besides the blood is sub- 

 jected to a tolerably strong pression. To these conditions already 

 so favourable to the osmotic exchanges are added electro motory 

 actions. This exceedingly interesting point of the physiology of 

 the capillaries has been elucidated by M. Becquerel in a series 

 of important papers. 1 If by means of a voltaic pile we decom- 

 pose water contained in a vessel divided into two compartments 

 by a membrane, we see the level rise in the negative compart- 

 ment. There is therefore material transport from the positive 

 pole to the negative pole. Something analogous takes place in 

 the capillaries. In consequence of the incessant chemical muta- 

 tions which are effected in the tissues and the vessels, electrical 

 currents are produced. The capillary vessel is electrised nega- 

 tively on the exterior, and positively in the interior of its wall. 

 The oxygen traverses the wall, and is deposited on the external 

 surf ace. As to the globules, they enter into contact with the posi- 

 tive internal wall. The oxygen, once infiltrated into the tissues, 

 oxydises them, gives rise to the production of carbonic acid, 

 which the electrical current tends to impel into the interior of 

 the capillary. In other terms, and leaving aside the old notion 

 of the two 'electric fluids, there is between the contents of the 

 capillary vessels and the histological elements of the tissues a 

 circular current, an electrical circuit, which carries the oxygen 

 to the outside of the vessels, and brings back the carbonic acid. 

 We shall have to expound in detail what relates to the special 

 office of the capillaries in the respiratory surfaces, and in the 

 liver ; we must content ourselves here with speaking of the 

 capillaries distinctively nutritive, of those instrumental in the 

 general exchanges between the blood and the tissues. But on 

 this point our work is already done, and it is sufficient to refer 

 the reader to the preceding chapters, in which nutrition has been 

 discoursed of in a general manner. We have indicated there 

 \\ hat substances come forth from the capillaries to be assimilated 

 by the anatomical elements, what others are yielded to the blood 

 1 Mi-moires d Comptes Rcndus de FAcad&nie des Sciences, from 1867 to 1870. 



