DIGESTION OF THE FOOD 25 



of splitting up the proteins into simpler compounds, 

 transforming in various ways the soluble nitrogen- 

 free parts of the food, and attacking the crude fibre 

 which is unacted upon by the various digestive 

 juices, and would otherwise undergo no change. 



Lactic and butyric acids are amongst the sub- 

 stances which result from the activity of the bacteria 

 upon the nitrogen-free extract and crude fibre. 

 In addition to these acids certain gaseous products, 

 such as carbon dioxide, marsh gas, and hydrogen, 

 are also formed. With fattening cattle and milch 

 cows the quantity of marsh gas formed daily is con- 

 siderable, as much as 700 litres (i litre = if pints) 

 having been collected from one animal. The 

 bacterial decomposition of the crude fibre renders 

 available many substances which would otherwise 

 not be acted upon by the digestive juices and so 

 would be lost. Bacteria, then, are of considerable 

 service in this respect, and they can be also very 

 useful to the animal in other ways. Some bacteria 

 have the power of forming proteins from certain 

 non-protein substances, probably with the assistance 

 of nitrogen-free extract substances; and such pro- 

 tein matter can be utilised exactly in the same way 

 as are the proteins of the food. Most probably 

 this transformation only takes place in the case of 

 the ruminants, for in this class the activity of the 

 bacteria is far greater than with animals which 

 have only a simple stomach like the horse or pig. 



