ABSORPTION BY BLOOD-VESSELS. 301 



The process by which digested materials are taken into the blood is called absorption. 

 It is now recognized that two sets of vessels are concerned in the performance of this 

 function; namely, the blood-vessels and the lacteals. Those parts of the food which 

 have been rendered fluid and are capable of forming a homogeneous mixture with the 

 blood-plasma are absorbed chiefly by the blood-vessels, although a small portion finds its 

 way into the lacteals. The emulsified fats are taken up in greatest part by the lacteals, 

 although a small quantity is taken directly into the blood. In treating of this subject, it 

 will be convenient to consider separately the action of these two kinds of vessels. 



Absorption by JBlood- Vessels. 



That soluble substances can pass through the delicate walls of the capillaries and 

 small veins and that absorption actually takes place in great part by blood-vessels, are 

 facts which hardly demand discussion at the present day. Soluble principles which 

 have disappeared from the alimentary canal have been repeatedly found in the blood 

 coming from this part, even when the lymphatics have been divided and communication 

 existed only through the blood-vessels. The old theoretical view which was entertained 

 before the lymphatics and lacteals were discovered was that absorption took place by 

 blood-vessels ; but, after special absorbent vessels had been described, it was generally 

 supposed that they furnished the only avenue for the entrance of new matters into the 

 economy, although the doctrine of vascular absorption was retained by a few. It was 

 only after the conclusive experiments of Magendie, in 1809, that positive proof was given 

 of the absorbing power of the blood-vessels. These experiments settled the question of 

 vascular absorption, although they led some to take too exclusive a view of the impor- 

 tance of the venous radicles in this function and to deny that absorption took place to any 

 considerable extent through the lymphatic and the lacteal system. At the present day, 

 there is no difference of opinion among physiologists concerning the direct absorption of 

 nutritive matters by the blood-vessels of the alimentary canal. It has been repeatedly 

 shown, indeed, that, during absorption, the blood of the portal vein is rich in albumi- 

 noids, sugar, and in other principles resulting from digestion. 



In the mouth and oesophagus, the sojourn of alimentary principles is so brief and 

 the changes which they undergo so slight, that no absorption of any moment can take 

 place. It is evident, however, that the mucous membrane of the mouth is capable ot 

 absorbing certain soluble matters, from the effects which are constantly observed when 

 the smoke or the juice of tobacco is retained in the mouth, even for a short time. In 

 the stomach, however, the absorption of certain materials takes place with great activity. 

 A large proportion of the ingested liquids and of those principles of food which are dis- 

 solved by the gastric juice and converted into albuminose is taken up directly by the 

 blood-vessels of the stomach. It may, indeed, be assumed, as a general law, that di- 

 gested matters are in great part absorbed as soon as their transformations in the alimen- 

 tary canal have been completed. 



In the passage of the food down the intestinal canal, as we have already seen, there 

 is a constant loss of material. As the digestion of the albuminoids is completed, these 

 principles are absorbed, and their passage into the mass of blood is indicated chiefly by 

 an increase in its proportion of albuminoid constituents. Many of the other products of 

 digestion, such as glucose and fatty emulsion, have also been demonstrated in quantity 

 in the blood of the portal vein during absorption. The fats, though taken up in greatest 

 part by the lacteals, are always found in greater or less quantity in the portal blood. It 

 has frequently been observed that, after a full meal consisting largely of fat, the blood 

 from the portal vein, as it cools and coagulates, leaves a white scum of fat upon the sur- 

 face. On one occasion, we observed, in the portal blood of an animal killed in full diges- 

 tion, a layer of fat on cooling so thick that a quantity of blood, which was spilled upon 

 a table and the floor, was white, like milk. We have since frequently attempted to 



