348 OF DIGESTION AND NUTRITIVE ABSORPTION. 



the lacteals ; and it appears strictly analogous to the property which is pos- 

 sessed by the different cells of Plants, of selecting, from the pabulum common 

 to all of them, the materials requisite for the elaboration of their own peculiar 

 products, such as colouring matter, starch, oil, &c. 



463. On the other hand, the Veins seem to be less concerned in nutritive 

 absorption, but take up from the alimentary canal a portion of almost any fluid 

 matters which it may contain. This seems to have been established by the 

 carefully conducted experiments of MM. Tiedemann and Gmelin, who mingled 

 with the food of animals various substances, which, by their colour, odour, or 

 chemical properties, might be easily detected in the fluids of the body. After 

 some time the animal was examined ; and the result was, that unequivocal 

 traces of the substances were not unfrequently detected in the venous blood 

 and in the urine ; whilst it was only in a very few instances, that any indi- 

 cation of them could be discovered in the chyle. The colouring matters em- 

 ployed were various vegetable substances ; such as gamboge, madder, and 

 rhubarb : the odorous substances were camphor, musk, assafoetida, &c. ; while, 

 in other cases, various saline bodies, such as muriate of barytes, acetate of lead 

 and of mercury, and some of the prussiates, which might easily be detected 

 by chemical tests, were mixed with the food. The colouring matters, for the 

 most part, were carried out of the system, without being received either into 

 the veins or lacteals ; the odorous substances were generally detected in the 

 venous blood and in the uterine, but not in the chyle ; whilst of the saline 

 substances many were found in the blood and in the urine, and a very few 

 only in the chyle. A similar conclusion might be drawn from the numerous 

 instances in which various substances introduced into the intestines have been 

 detected in the blood, although the thoracic duct had been tied ; but these 

 results are less satisfactory, because even if there is no direct communication 

 (as maintained by many) between the lacteals and the veins in the mesenteric 

 glands, the partitions which separate their respective contents are evidently so 

 thin, that transudation may readily take place through them. It would seem 

 probable, that substances perfectly dissolved in the fluids of the stomach, are 

 taken into the blood-vessels so copiously distributed on its walls, by the simple 

 and necessary process of Endosmose ; in this manner we may account for the 

 fact, that saline substances are for the most part readily absorbed into the 

 blood ; and there seems reason to believe that the albuminous portion of the 

 chyme, together with the saccharine principles or the products of their trans- 

 formation, may thus be introduced directly into the circulating current, without 

 passing through the lacteals. On this subject there is much need of further 

 information. 



VII. Absorption by the General Surface. 



464. The Mucous Membrane of the alimentary canal is by no means the 

 only channel through which nutritive or other substances may be introduced 

 into the system. In the lowest tribes of animals, and in the earliest condition 

 of the higher, it would seem as if Absorption by the external surface is almost 

 equally important to the maintenance of life, with that which takes place 

 through the internal reflexion of it forming the walls of the digestive cavity. 

 In the adult condition of the higher animals, however, the special function of 

 the latter is so much exalted, that it usually supersedes the necessity of any 

 other supply; and the function of the cutaneous and pulmonary surfaces may 

 be considered as rather that of exhalation than of absorption. But there are 

 peculiar conditions of the system, in which the imbibition of fluid through these 

 surfaces is performed with great activity, supplying what would otherwise be 

 a most important deficiency. It may take place either through the direct 



