E.ESPIRATION. 59 



The absorption that takes place through the skin, or accidental 

 absorption,, as it is called, is effected by the agency of the lymphatics 

 also, which are here very abundantly distributed. In this case there 

 is no selection, the fluids passing in by simple imbibition. In the 

 pulmonary mucous membrane, as in the digestive, thin fluids are 

 taken up by the veins. In general, wherever a thin fluid is placed in 

 contact with an extended surface, it will be taken up by those vessels 

 that present the largest surface and the thinnest walls. The rapidity 

 of this absorption through the skin and mucous membrane, is greatly 

 influenced by the condition of the blood-vessels ; being very great 

 when the vessels are drained of their contents, and proportionately 

 small when they are full. The presence, or absence of the epider- 

 mis affects it as might be readily inferred. The external integument, 

 so long as it is covered with the epidermis, absorbs with extreme slow- 

 ness ; the epidermis removed, however, — by means of a blister, — 

 and the vascular lamina of the corium exposed, absorption goes on 

 with extreme rapidity. It is upon this fact that what is called the 

 endermic method of exhibiting medicines depends. 



In regard to the absorption of fluids from serous cavities it is 

 remarked, that imbibition does not explain all the phenomena ; the 

 probability is that the lymphatics under certain circumstances are 

 also concerned. If the fluid in the veins be more concentrated than 

 those to be absorbed, then imbibition is sufficient. But, if, on the 

 contrary, the external fluid is equally concentrated with that con- 

 tained in the interior of the vessels, the two fluids ought to pass 

 through the membrane in both directions with equal rapidity. And 

 if the fluid in the vessels be less concentrated of the two, it will exude 

 in greater quantity so as to increase the amount of the eflused fluid. 

 A collection of fluids in the pleura, or peritoneum, containing albu- 

 men and salts in the same state of concentration as these substances 

 exist in the blood, would not be diminished in quantity by imbibition 

 alone ; there would be merely an interchange of the saline matters 

 contained in the external fluid and in the blood, while the bulk of the 

 former would remain the same. The probability is, therefore, that 

 in this case there would be other agents than the blood-vessels con- 

 cerned in the absorption, viz: the lymphatics. It is possible that the 

 process of endosmosis may be modified by a peculiar attraction 

 exerted by the tissues on the fluids circulating in them ; an attraction, 

 by the agency of which the fluids in the tissues may be retained 

 while the external fluid is absorbed, so that merely absorption, and 

 not an interchange of fluids, as is the case under ordinary circum- 

 stances, is the result. 



RESPIRATION. 



Respiration is that function by which venous blood is converted 



