512 OF ABSORPTION AND SANGUIFICATION. 



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 indication of them could be discovered in the chyle. The colouring 

 matters employed were various vegetable substances ; such as gamboge, mad- 

 der, 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 re- 

 ceived either into the veins or lacteals ; the odorous substances were gene- 

 rally detected in the venous blood and in the urine, 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 respect- 

 ive 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 dis- 

 tributed 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 prin- 

 ciples or the products of their transformation, may thus be introduced directly 

 into the circulating current, without passing through the lacteals. On this 

 subject there is much need of further information. 



2. Absorption from the Body in general. 



676. 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 circulating apparatus. The Lymphatic system is present in all animals 

 which have a lacteal system ; and the two evidently constitute one set of ves- 

 sels. The lymphatics, however, instead of commencing on the intestinal 

 walls, are distributed through the greater part of the body, especially on the 

 Skin ; their origins cannot be clearly traced ; but they seem in general to form 

 a plexus in the substance of the tissues, from which the convergent trunks 

 arise. After passing, like the lacteals, through a series of glandular bodies 

 (the precise nature of which will be presently considered, 682), they empty 

 their contents into the same receptacle with the lacteals ; and the mingled 

 products of both pass into the Sanguiferous system. We find in the Skin, 

 also, a most copious distribution of capillary blood-vessels, the arrangement of 

 which is by no means unlike that of the blood-vessels of the alimentary canal ; 

 and its surface is further extended by the elevations that form the sensory 

 papillae, which are in many points comparable to the intestinal villi, although 

 their special function is so different. In the lowest tribes of animals, and in 

 the earliest condition of the higher, it would se^m as if Absorption by the ex- 

 ternal 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, how- 



