118 IMIYSIOLOCY OF THE CIRCULATION. 



and mediately ; viz., by increasing the rapidity, and 

 facilitating the passage of the streams of blood tra- 

 versing the veins. 



As giving a concise view of the general action of 

 the causes producing clFu^ion and absorption, it may 

 be stated that effusion is promoted, 1st, directly, by 

 whatever increases the amount of pressure acting on 

 the internal surface of the blood-vessels ; and 2ndly, 

 indirectly, by whatever diminishes the pressure acting 

 on their exterior : while absorption is rendered 

 more active, 1st, by whatever directly increases the 

 amount of external pressure ; and 2udly, by whatever 

 increases the rapidity of the currents of blood ; and 

 thus diminishes the pressure acting on the internal 

 surface of the absorbing blood-vessels. 



1'ART III. 



IJEI.N<: AN ATTEMPT TO INQUIRE HOW FAR THE ENTRANCE "I 

 CERTAIN GASES INTO, AND THL DISCHARGE OF OTHERS 

 FROM, THE INTERIOR OF THE HLOOD-VESSELS, MAY BE 

 ACCOUNTED FOR ON THE GENERAL PRINCIPLES OF EFFUSION 

 AND ABSORPTION. 



IT now only remains to inquire how far the same 

 views are applicable to the explanation of those 

 functions in animals which consist in the extrication 

 and discharge of certain gases from, and the absorp- 

 tion of others into, the mass of circulating blood. 

 Here the doctrine of capillary attraction is confessed 



