88 THEORY OF FEEDING. 



of the woody fibre is broken up in the large intestine by 

 the action of bacteria which are brought into the body 

 along with the food ; the result being that the woody fibre 

 which undergoes this change becomes converted into car- 

 bonic acid, marsh gas, acetic acid, and butyric acid. Bunge 

 explains that this carbonic acid is absorbed by the intestines 

 and is breathed out by the lungs ; but that the marsh gas 

 remains in the intestines until it is passed out in the form of 

 flatus. 



As the products of this reaction cannot be utilised by the 

 system, this breaking up of woody fibre by bacteria must not 

 be regarded as digestion. 



WATER FOR THE DIGESTIVE JUICES. 



We have seen that for every 2^ Ib. of hay which a horse 

 eats, his salivary glands have to pour into his mouth a gallon 

 of saliva (10 Ib.) ; and for the same weight of oats, more than 

 a quart of saliva. Hence, 6 gallons of saliva would be 

 required for the mastication of a daily ration of 12 Ib. of 

 hay and 1 1 Ib. of oats, without counting the saliva which is 

 continually secreted while the animal is not feeding. There 

 are also large quantities of gastric juice, pancreatic juice, bile, 

 and intestinal juice secreted. Considering the composition of 

 these digestive fluids, we may safely say that the water con- 

 tained in their daily supply to the alimentary canal, is on an 

 average a good deal more than double the quantity of water 

 drunk by the horse in the twenty-four hours. It is therefore 

 evident, especially when we note the large expenditure of 

 water from the skin, lungs, and kidneys, that a considerable 

 portion of the water contained in the outpoured digestive 

 fluids becomes reabsorbed before it leaves the system. I need 

 hardly explain that all the water in the digestive juices is 

 derived from the blood. This question of the reabsorption of 

 water does not affect the fact that the animal needs his blood 



