398 VETERINARY PHYSIOLOGY 



3rd. That it should contain sufficient proteins to make good 

 the waste of these substances. 



1. Diet must be capable of Digestion, Absorption, and 

 Assimilation. The constituents of the food, to be of use in 

 the body, must be capable of being acted on by the digestive 

 juices. For this reason young vegetables, in which the cells 

 have only a thin capsule of cellulose, which is easily broken up 

 in mastication or by maceration in the rumen of the ox or 

 colon of the horse, yield more of their nutritious constituents 

 than do the older vegetables, where the capsule has become 

 indurated, and possibly in part converted to lignin and other 

 encrusting substances. A food that is capable of proper 

 digestion is also capable of absorption, provided the cells lining 

 the intestine are active. 



Lastly the food, to be of use, must consist of proteins, 

 carbohydrates, and fats, which alone are capable of being taken 

 up assimilated by the tissues, so that they can be broken 

 down and their energy liberated. Various matters may be 

 absorbed from the food into the blood merely to be again 

 excreted by the kidneys, without serving any purpose in 

 the body. 



The investigation of the extent to which food is digested is 

 more difficult in herbivorous than in carnivorous animals, 

 because the process of digestion is so much slower. In the 

 ruminants it takes nearly five days for the contents of the 

 alimentary canal to be cleared out. In the horse it takes 

 about four days. 



To determine the extent to which digestion has been carried 

 out, analyses of the food and of the faeces during a somewhat 

 prolonged period have to be made. It has been found that 

 digestion of various plant foods goes on to about the same 

 extent in the various ruminants, but that in the horse it is 

 less complete. When fed on hay, 11 or 12 per cent, less of 

 the solids are digested in the horse than in the sheep. The 

 proteins are digested about as well, the fats much less well 

 digested, sometimes as much as 50 per cent, less well ; the 

 carbohydrates about 10 per cent, less well. The crude fibre of 

 the hay, which is well digested in the ruminants, is not so 

 perfectly dissolved in the horse. 



Some examples of the extent to which some of the commoner 



