FUNCTIONS OF VEGETATIVE LIFE. 365 



action has ceased, it is taken up (as it were) by the muscular coat of the oeso- 

 phagus itself, and is conveyed into the stomach. How far the movements of the 

 lower parts of the oesophagus and of the stomach are in Man dependent upon 

 reflex action, is uncertain; the facts which have been ascertained on this point, 

 by experiment on animals, will be detailed in their proper place ( 428, 430). 



378. In the Stomach, the food, certain components of which have been 

 already altered by the chemical action of the saliva, is brought Bunder the influ- 

 ence of the gastric secretion; the chemical action of which, aicled by the con- 

 stantly-elevated temperature of the interior of the body, and by the continual 

 agitation effected by the contractions of the parietes of the organ, effects a 

 more or less complete reduction of it. Some of its nutritive components, being 

 actually dissolved by the gastric juice, are thus prepared for immediate absorp- 

 tion; but others require the admixture of the biliary and pancreatic secretions, 

 whereby various changes are effected in their condition, which prepare them 

 also for reception into the circulating system. The nutritious portion being 

 gradually taken up by the Bloodvessels and by the Absorbent vessels (or Lac- 

 teals), which are distributed on the walls of the alimentary canal, the indigesti- 

 ble residue is propelled along the intestinal tube by the simple contractility of 

 its walls, undergoing at the same time some further change, by which the nu- 

 tritive materials are still more completely extracted from it. And at last, the 

 excrementitious matter, consisting not only of the insoluble portion of the food 

 taken into the stomach, but also of part of the secretion of the liver, and of that 

 of the mucous surface of the intestines and of their glandulge, is voided from 

 the opposite extremity of the canal, by a muscular exertion, which is partly 

 reflex, like that of deglutition, but is partly voluntary, especially (as it would 

 appear) in Man. The whole of this series of operations, by which the nutri- 

 tive materials are prepared for being absorbed, may be considered as constituting 

 the function of Digestion. 



379. The introduction of the nutritive materials thus prepared, into the ves- 

 sels which convey them to the tissues, constitutes the function of Absorption. 

 But these materials undergo important changes in their progress towards the 

 centre of the circulation, whereby they are brought more nearly to the condition 

 of true Blood; and these changes are designated by the term Assimilation. 

 There seems no doubt that fluid containing saline, albuminous, or other matters 

 in a state of complete solution, may be absorbed by the Bloodvessels with which 

 the mucous membrane of the alimentary canal is so copiously supplied ; and this 

 simple process of imbibition probably takes place according to the physical laws 

 of Endosmose. But the selection and absorption of some of the nutritive mate- 

 rials appear to be performed, not by vessels, but by the specific vital endowments 

 of cells ( 460), which subsequently yield up their contents to the Lacteals. 

 The fluid thus absorbed, which now receives the name of Chyle, is propelled 

 through the Lacteals by the contractility of their walls ; aided in part, perhaps, 

 by a vis a tergo derived from the force of the absorption itself. With the recep- 

 tion of the nutritious fluid into the vessels, commences its real preparation for 

 Organization. Up to that period, it cannot be said to be in any degree vital- 

 ized; the changes which it has undergone being only of a chemical and physical 

 nature, and such as merely prepare it for subsequent assimilation. But in the 

 passage of that which has been taken up by the Bloodvessels, through the 

 Liver, very important changes are effected in its condition, whereby it is brought 

 to a state more nearly corresponding with true Blood. And in like manner, the 

 Chyle, in passing through the long and tortuous system of Absorbent vessels 

 and glands, undergoes changes which, with little chemical difference, manifest 

 themselves by a decided alteration in its properties ; so that the chyle of the 

 Thoracic duct is evidently a very different fluid from the chyle of the Lacteals, 

 approaching much nearer to blood in its general characters. These characters 



