724 PHYSIOLOGY 



digestion is not, however, carried out by the action of juices secreted 

 by the intestinal canal itself. The solution of the cellulose is effected 

 partly under the influence of a cellulase or cytase present in the 

 plant cells themselves, partly under the influence of the micro- 

 organisms living in the paunch or caecum. Under the influence of 

 these bacteria cellulose is dissolved with the production of carbon 

 dioxide, methane, and butyric and acetic acids. In man the greater 

 part of the cellulose of the food is undigested, and its chief value 

 appears to be that of lending bulk to the indigestible material and 

 so aiding the normal movements of the intestines. In the case of 

 young plant cells, such as those of lettuce or carrots, a certain per- 

 centage of the cellulose undergoes solution in the intestine. Here, 

 again, the digestion is probably effected by the agency of putrefactive 

 organisms. 



ALCOHOL. When alcohol is taken by man in moderate quantities 

 the greater part of it undergoes oxidation, and leaves the body as 

 carbon dioxide and water. About 10 per cent, which escapes oxida- 

 tion is excreted unaltered by the lungs and urine. This oxidation 

 of alcohol is a result of true utilisation, since the addition of a certain 

 amount of alcohol to the food does not result in an increase of the 

 output of carbon dioxide. In small quantities therefore alcohol 

 can act as a food. This function, however, is quite unimportant, 

 and is overshadowed by the poisonous action of this substance. A 

 man unaccustomed to its action cannot take more than 16 to 25 grm. 

 without experiencing its poisonous effects. This amount of alcohol 

 only represents a total heat-value of 112 to 175 calories, i.e. only 

 about 5 per cent, of the total energy requirements of the body. 

 Only very rarely therefore can we be justified in administering alcohol 

 as a food. Its value in a diet is entirely that of an accessory or 

 adjuvant in exciting appetite by its taste and smell, an advantage 

 which is largely counterbalanced by the danger of introducing a 

 poison into the body which on long continuance tends to set up 

 various degenerative changes in the tissues, and if taken in any 

 quantity at one time causes a temporary abolition of those processes 

 of inhibition and control which have been the determining factors 

 in the survival of the race throughout the struggle for existence. 



THE INORGANIC FOOD-STUFFS 



If an animal be fed with the proper quantities of fats, proteins, 

 and carbohydrates, from which all the salts have been removed 

 as completely as possible, it rapidly shows a distaste for the food, 

 becomes ill, and dies in a shorter time than if it were receiving no 

 food at all. Part of the symptoms which occur in these cases are 

 due to the production of acid substances, e.g. sulphuric acid, in the 



