6T8 ABSORPTION. 



and lymphatic vessels. The anatomy of the venous system will be 

 given under Circulation. 



1. PHYSIOLOGY OF VENOUS ABSORPTION. 



Whilst the opinion prevailed universally, that the lymphatics are the 

 sole agents of absorption, the fluid, circulating in the veins, was con- 

 sidered to consist entirely of the residue of arterial blood, after it had 

 passed through the capillary system, and been subjected to the different 

 nutritive processes. We have seen, however, that drinks are absorbed 

 by the mesenteric veins; and we shall hereafter find, that various other 

 substances enter the venous system. It is obvious, therefore, that ve- 

 nous blood cannot be simply the residue of arterial blood; and we can 

 thus account for the greater capacity of the venous than of the arterial 

 system. The facts, which were referred to, when considering the ab- 

 sorption of fluids from the intestinal canal, may have been sufficient 

 to show, that veins are capable of absorbing; as odorous and colouring 

 properties of substances were distinctly found in the mesenteric veins. 

 A question arises, whether any vital elaboration is concerned, as in the 

 case of the chyle, or whether the fluid, when it attains the interior of 

 the vessel, is the same as without? M. Adelon, 1 who, with many of 

 the German physiologists, believes in both venous and lymphatic ab- 

 sorption, and venous and chyliferous absorption, conceives, that a vital 

 action takes place at the very extremities of the venous radicles, pre- 

 cisely similar to that which is presumed to be exerted at the extremities 

 of the lymphatic and chyliferous radicles. In his view, consequently, 

 an action of elaboration is exerted upon the fluid, which becomes, in all 

 cases, converted into venous blood at the very moment of absorption. 

 On the other hand, MM. Magendie, 2 FodeYa, 3 and others maintain, that 

 the substance when possessed of the necessary tenuity soaks through the 

 vessel ; and that this act of imbibition is purely physical. In their view, 

 consequently, the fluid within the vessel must be the same as without. 



In favour of the vital action of the veins we have none of that evi- 

 dence, which strikes us in the case of the chyliferous and lymphatic 

 vessels. In the last we invariably find fluids, identical in all essen- 

 tial respects in physical characters ; and never containing extraneous 

 matter, if we make abstraction of certain salts, that have been occa- 

 sionally met with in the thoracic duct. In the veins, on the other hand, 

 the sensible properties of odorous and colouring substances have been 

 frequently apparent. It may be remarked, however, that the fluid, 

 flowing in the veins, is as identical in composition as the chyle or the 

 lymph. This is true ; but it must be recollected, that the greater part 

 of it is the residue of arterial blood ; and that its hue and other sen- 

 sible properties are such as to disguise any absorbed fluid, not itself 

 possessing strong characteristics. The fact, now indisputable, that 

 various substances, placed outside the veins, have been detected in the 

 blood within, is not only a proof that the veins absorb ; but that no 



1 Art. Absorption, in Diet, de Medecine, 2de edit., i. 239, Paris, 1832 ; and Physiologie de 

 1'Homme, 2de edit, iii. 113, Paris, 1829. 



2 Precis, &c., 2de edit., ii. 271. 



3 Recherches Experimentales sur 1'Exhalation et 1'Absorption, Paris, 1823. 



