ELASTICITY OF THE COATS OF VESSELS. 153 



dilated condition, and thus are gradually produced the 

 well-known forms of ecstasis, such as we are familiar 

 with in the arteries under the name of aneurysms, and 

 in the veins under that of varices. In these processes, 

 we have not so much, as has been represented of late, 

 to deal with primary disease of the inner coat, as with 

 changes which are situated in the elastic and muscular 

 middle coat. 



If therefore it is the muscular elements of the arteries 

 that have the most important influence upon the quan- 

 tity of blood to be distributed, and the mode of its dis- 

 tribution, in the several organs, and the elastic elements 

 that are chiefly concerned in the production of a rapid 

 and equable stream, they nevertheless exercise only an 

 indirect influence upon the nutrition of the parts which 

 lie outside the vessels themselves, and in this matter, we 

 are obliged to betake ourselves, as a last resource, to the 

 simple, homogeneous membrane of the capillaries, without 

 which indeed not even the constituents of the walls of 

 the larger vessels provided with vasa vasorum would be 

 able to maintain themselves for any lengthened period. 

 The difficulty which here presents itself has, as you 

 know, during the last ten years, been chiefly got over by 

 the assumption of the existence of diffusive currents 

 (endosmosis and exosmosis) between the contents of the 

 vessels and the fluid in the tissues ; and by regarding the 

 capillary wall as a more or less indifferent membrane, 

 forming merely a partition between two fluids, which 

 enter into a reciprocal relation with one another ; while 

 the nature of this relation would be essentially deter- 

 mined by the state of concentration they are in and their 

 chemical composition, so that, according as the internal 

 or the external fluid was the more concentrated, the dif- 

 fusive stream would run inwardly or outwardly, and, 



