264 DIGESTION. 



and the disease which is here so remarkably evident in its 

 symptoms, causes, and cure, is matched by numberless 

 other ailments, the causes of which, however, although 

 analagous, are less exactly known, and therefore less 

 easily combated. 



With regard to the quantity, too, as well as the kind of 

 food necessary, there will be much diversity in different 

 individuals. Dr. Dalton believed, from some experiments 

 which he performed that the quantity of food necessary 

 for a healthy man, taking free exercise in the open air, is 

 as follows : 



Meat 1 6 ounces, or I 'oo Ib. avoird. 



Bread . . . . . 19 1-19 



Butter or Fat . . . . 3i 0-22 



"Water 52 fluid ozs.,, 3-38 



The quantity of meat, however, here given is probably 

 more in proportion to the other articles of diet enumerated 

 than is needful for the majority of individuals under the 

 circumstances stated. 



PASSAGE OF POOD THROUGH THE ALIMENTARY CANAL. 



The course of the food through the alimentary canal of 

 man will be readily seen from the accompanying diagram 

 (fig. 66). The food taken into the mouth passes thence 

 through the oesophagus into the stomach, and from this 

 into the small and large intestine successively ; gradually 

 losing, by absorption, the greater portion of its nutritive 

 constituents. The residue, together with such matters as 

 may have been added to it in its passage, is discharged 

 from the rectum through the anus. 



We shall now consider, in detail, the process of diges- 

 tion, as it takes place in each stage of this journey of the 

 food through the alimentary canal. 



The Salivary Glands and the Saliva. 



The first of a series of changes to which the food is sub- 

 jected in the digestive canal, takes place in the cavity of 



