CHAPTEE I. 

 THE TISSUES AND MECHANISMS OF DIGESTION. 



THE food in passing along the alimentary canal is subjected to the 

 action of certain juices which are produced by the secretory activity 

 of the epithelium-cells lining the canal itself or forming part of its 

 glandular appendages. These juices (viz. saliva, gastric juice, 

 bile, pancreatic juice, and the secretions of the small and large 

 intestines), poured upon and mingling with the food, produce 

 in it such changes, that from being largely insoluble it becomes 

 largely soluble, or otherwise modify it in such a way that the 

 larger part of what is eaten passes into the blood, either directly 

 by means of the capillaries of the alimentary canal or indirectly 

 by means of the lacteal system, while the smaller part is discharged 

 as excrement. 



We have therefore to consider First, the properties of the 

 various juices, and the changes they bring about in the food eaten. 

 Secondly, the nature of the processes by means of which the 

 various epithelium-cells of the various glands and various tracts of 

 the canal are able to manufacture so many various juices out of 

 the common source, the blood, and the manner in which the 

 secretory activity of the cells is regulated and subjected to the 

 needs of the economy. Thirdly, the mechanisms, here as elsewhere 

 chiefly of a muscular nature, by which the food is passed along the 

 canal, and most efficiently brought in contact with successive 

 juices. Fourthly and lastly, the means by which the nutritious 

 digested material is separated from the undigested or excremental 

 material, and absorbed into the blood. 



