SECRETION AND EXCRETION. 181 



alkaline fluid, which contains, besides other substances, a peculiar 

 resinous principle. Unlike other secretions, it is formed from the 

 venous blood. The whole veins of the stomach and intestines, 

 instead of going directly to the right side of the heart, first unite to 

 form one great trunk (vena portae), which divides, like an artery, in 

 the substance of the liver; and these branches, by which the bile 

 is secreted, again unite, and join the veins going to the heart in the 

 ordinary way. In some species the veins going to the kidneys have 

 a similar distribution. From this, and for various other reasons, it is 

 strongly conjectured that the liver assists the lungs in purifying the 

 venous blood, by depriving it of a portion of its carbon ; and, 

 accordingly, we always notice the liver larger in animals in propor- 

 tion as the activity of their lungs diminishes. The carbon unites 

 with oxygen, and forms carbonic acid in the lungs, but it seems to 

 escape from the liver in union with another gas called hydrogen, 

 forming the resinous and other principles of the bile. We have 

 before stated that less oxygen is consumed, and of course less car- 

 bonic acid is produced, when the temperature is high, than when it 

 is low. Hence, probably, a chief cause of the diseases of the 

 liver we are liable to in warm climates ; for if less carbon be given 

 off at the lungs, more will have to be secreted by the liver ; and 

 any part required to do more than its ordinary duty, is liable to 

 become deranged. It is thought about six or eight ounces of bile 

 are ordinarily secreted daily. Another analogous substance called 

 urea, is secreted by the kidneys, which are glands that also proba- 

 bly assist in purifying the blood. It is probable that both the 

 resinous matter in the bile, and the urea in the urine, exist ready 

 formed in the blood, and are merely separated by their respective 

 glands ; as, when the kidneys of dogs have been taken away, urea 

 has been detected in the blood, which could not be the case if the 

 kidneys formed it. It sometimes happens, especially in drunkards, 

 that one or both of these glands become diseased, and are incapable 

 of separating the peculiar fluids mentioned; and then these, being 

 retained in the system, act as poisons, producing insensibility and 

 death. In the case of the liver, this forms one cause of jaundice ; 

 but jaundice is more commonly caused by an obstruction to the flow 

 of the bile through its ducts. The passage of gall-stones (which 

 are only bile solidified) from the gall-bladder through the gall or 

 common ducts, is a common cause of obstruction. When the sub- 

 stance of the liver becomes diseased, the flow of blood through its 



