3o0 THE VITAL FUNCTIONS. 



obstacle to the advancement of our knowledge 

 in this department of physiology. Almost the 

 only result, therefore, which can be collected 

 from these laborious researches in microscopic 

 anatomy, is that nature has employed a great 

 diversity of means for the accomplishment of 

 secretion ; but we still remain in ignorance as to 

 the kind of adaptation, which must assuredly 

 exist, of each stnicture to its respective object, 

 and as to the nice adjustment of chemical affinities 

 which has been provided in order to accomplish 

 the intended effects.* Electricity is, no doubt, 

 an important agent in all these processes ; but 



* The only instance in which we can perceive a correspondence 

 between the chemical properties of the secretion, and the kind 

 of blood from which it is prepared, is in the liver, which, unlike 

 all the other glands, has venous, instead of arterial blood, sent 

 to it for that purpose. The veins, which return the blood that 

 has circulated through the stomach, and other abdominal viscera, 

 are collected into a large trunk, called the vena portce, which 

 enters the liver, and is there again subdivided and ramified, as if 

 it were an artery : its minuter branches here unite with those 

 of the hepatic artery, and ramify through the minute lobules 

 which compose the substance of the liver. After the bile is 

 secreted, and carried off by hepatic ducts, the remaining blood 

 is conducted, by means of minute hepatic veins, which occupy 

 the centres of each lobule, into larger and larger trunks, till they 

 all unite in the vena cava, going directly to the heart. (See 

 Kiernan's Paper on the Anatomy and Physiology of the Liver, 

 Phil. Trans, for 1833, p. 711.) A similar system of venous 

 ramifications, though on a much smaller scale, has been dis- 

 covered by Jacobson, in the kidneys of most fishes and reptiles, 

 and even in some birds. 



