302 ABSORPTION. 



demonstrate this excessively chylous condition of the blood during the absorption of fats, 

 but have found that it is not generally so well marked. 



The greatest part of the food is absorbed by the intestinal mucous membrane, and, 

 with the alimentary substances proper, a large quantity of secreted fluid is reabsorbed. 

 This fact is particularly striking as regards the bile. The biliary salts disappear as the 

 alimentary mass passes down the intestine and are undoubtedly absorbed, although they 

 are so changed that they cannot be detected in the blood by the ordinary tests. In this 

 portion of the alimentary canal, it will be remembered that an immense absorbing sur- 

 face is provided, by the arrangement of the mucous membrane in folds, forming the val- 

 vulae conniventes, and by the presence of the innumerable villi which are found through- 

 out the small intestine. A certain portion of the gaseous contents of the intestines is also 

 absorbed, although it is not easily ascertained what particular gases are thus taken up. 



Absorption by Lacteal and Lymphatic Vessels. 



The history of the discovery of what is ordinarily termed the absorbent system of 

 vessels, from the vague allusions of Hippocrates, Galen, Aristotle, and others, to the de- 

 scription of the thoracic duct in the middle of the sixteenth century, by Eustachius, and 

 finally to the discovery of the lacteals by Asellius, in 1622, is more interesting in an ana- 

 tomical than in a physiological point of view. Our knowledge of the anatomy of the 

 absorbent system dates from the discovery of the thoracic duct ; but, from the discovery 

 of the lacteals by Asellius, dates the history of these vessels as the carriers of nutritive 

 matters from the intestinal canal to the general system. 



In 1649, Pecquet discovered the receptaculum chyli and demonstrated that the lacteals 

 did not pass to the liver, but emptied the chyle into the commencement of the thoracic 

 duct, by which it was finally conveyed into the venous system. In 1650-'51, the ana- 

 tomical history of the absorbect vessels was completed by the discovery, by Rudbeck, 

 of vessels carrying a colorless fluid, in the liver and finally in almost all parts of the 

 body. Eudbeck demonstrated the anatomical identity of these vessels with the lacteals. 

 They were afterward carefully studied by Bartholinus, who gave them the name of lym- 

 phatics. It is unnecessary to follow out the various researches made into the structure 

 of the lymphatics in man and the inferior animals by the Hunters, Hewson, Monro, 

 Cruikshank, and other of the older anatomists and physiologists. 



The old idea, which dates from the discoveries of Asellius and Pecquet, that the lac- 

 teals absorb all the products of digestion, was overthrown by the experiments of Magen- 

 die and of those who experimented after him upon vascular absorption. It is now known 

 that the fatty portions of the food, reduced to a very fine emulsion by the pancreatio 

 juice, are absorbed by this system of vessels, and that these are the only principles which 

 are taken up in great quantity. The arguments which we have already mentioned are 

 sufficient to establish this fact. If the abdomen of a living animal be opened during full 

 digestion, then, and then only, will the lacteals and the thoracic duct be found dis- 

 tended with fatty emulsion. If the organ which digests fat be rendered incapable of 

 performing its function, the lacteals cease to carry chyle. These vessels do not appear 

 in the mesentery until the food has passed the orifice of the pancreatic duct. Finally, 

 the observations of Bouchardat and Sandras remove all doubt as to the absorption of the 

 products of the digestion of fatty matters by the lacteals ; for these observers found not 

 only that in dogs the proportion of fat in the chyle was increased pari passu with an in- 

 crease in the quantity of fat taken as food, but that the particular kinds of fat adminis- 

 tered to the animals could be recognized in the chyle. We have seen that a certain quan- 

 tity of fat escapes the lacteals and is absorbed directly by the blood-vessels ; and it be- 

 comes an important question to determine whether the lacteals, in addition to their more 

 prominent function, be not concerned in the absorption of drinks, the albuminoids, saline 

 and saccharine matters, etc. This question will be taken up after a consideration of 

 certain points in the anatomy of the lymphatic system. 



