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oxygen; on reaching its final distribution in the tissues, it effects 

 their oxydation, producing heat; and as it loses its oxygen and re- 

 ceives the metamorphosed products of the tissues, it takes the blue 

 colour of the venous blood. Now contrast the relations of arterial 

 and venous blood to the tissue. The former must have an intense 

 affinity for these, since it can oxydize them; but the latter an attrac- 

 tion which is correspondingly less. Upon the principle stated above, 

 therefore, motion will take place from the arterial to the venous 

 side. The systemic circulation, therefore, is due to the deoxydation 

 of arterial blood. 



2c?. To the Pulmonary Circulation. — Here venous blood reaches 

 the walls of the air-cells, and, taking oxygen from the air, becomes 

 arterialized. Now venous blood has a high affinity for oxygen, as is 

 shown by its absorbing that gas; but when the change has taken 

 place the affinity is satisfied. The change from venous to arterial 

 blood, which takes place on the air-cells charged with oxygen, ought, 

 upon these principles, to be accompanied by a movement from the 

 venous to the arterial side. The pulmonary circulation, therefore, 

 is due to the oxydation of venous blood, and ought to be from the 

 venous to the arterial side. 



3d. To the Portal Circulation. — Two forces are at work to drive 

 the portal blood out of the liver into the ascending cava. The portal 

 blood is acted on by the liver for the separation of bile. The affini- 

 ties acting towards this end being satisfied, the residue of the blood 

 passes over into the hepatic veins. Between the portal blood and 

 the liver there is an energetic affinity, shown by the chemical decom- 

 position which takes place. That change effected the residue forms 

 the venous blood of the hepatic veins. The second force is after the 

 blood of the hepatic artery, having served for the economic purposes 

 of the liver, is thrown into the portal plexus. The pressure of the 

 arterial blood in the hepatic capillaries upon this is sufficient, not 

 only to impel it into the capillaries of the portal veins, but also to 

 give it a pressure in a direction towards the hepatic veins. No 

 regurgitation can take place backwards through the portal vein, 

 upon the boold arriving from the chylopoietic viscera, because along 

 that channel there is a pressure in the opposite direction, arising 

 from the arterial blood of the aortic branches. The pressure, there- 

 fore, arising from the relations of the hepatic arterial blood conspires 

 with that arising from the portal blood, and both together join in 

 giving rise to motion towards the ascending cava. 



Wi. To the Placental Circulation. — The umbilical arteries take 

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