THE ANIMAL BODY DIGESTIONMETABOLISM 23 



40. Stomach digestion of ruminants. Tho the first three stomachs of 

 ruminants secrete no enzymes, but only water, they are highly important 

 in digestion. The nutrients of plants are enclosed within the cellulose 

 cell walls, and where these are hard and thick, as in hay and straw, the 

 digestive fluids can not easily reach and attack the nutrients locked with- 

 in. As we have seen, when ruminants swallow solid food, it passes chiefly 

 into the paunch. Here it is softened by the moisture, slowly but thoroly 

 mixed by muscular contractions, and ground against the rough lining. 

 All this prepares the food for easy digestion further on. 



A considerable amount of actual digestion also occurs in these first 

 stomachs, especially in the paunch, thru the action of certain bacteria. 

 These bacteria attack the cellulose and pentosans of the feed and break 

 them down with the production of heat and the formation of organic 

 acids, chiefly acetic acid and butyric acid, and of gases, including 

 methan (marsh gas), carbon dioxid, and hydrogen. The acids serve 

 as food, the same as do the sugars, but the gases are useless and are 

 excreted. This digestive action is highly important, for a large part 

 of hay and other roughages consists of cellulose and pentosans (9), 

 and Nature has provided no enzymes of the digestive tract which are able 

 to digest these compounds. The ability of the animal to use them as 

 food therefore depends on the fermentations caused by these bacteria. 

 In this action the cell walls of the plant materials are broken down, 

 setting free the nutrients contained within, so they can be more easily 

 acted on by the digestive juices in the true stomach and the intestines. 



Not only do the bacteria digest cellulose and pentosans, but also they 

 may attack starch and sugar. This action is detrimental, for these 

 nutrients would be digested more efficiently later on in the small intes- 

 tine, while in the bacterial digestion a considerable part of their feeding 

 value is lost thru the heat and gases produced in the fermentations. (84) 

 When fresh, easily fermented forage, such as green clover or alfalfa, 

 is eaten, the bacterial action may then be so great that gas is produced 

 faster than it can be carried away, and " bloat " results. (340) 



The fact that only a small amount of ptyalin is present in the saliva 

 of the ox and sheep, or that it is entirely absent, as is claimed by some, 

 is thus advantageous. If their saliva easily converted starch into sugar, 

 a large amount of sugar would be formed in the paunch, which would 

 then in turn be attacked by bacteria, with much resultant loss of nutri- 

 ment. In addition to the digestion in the paunch caused by bacteria, 

 more or less is also undoubtedly effected by the enzymes which are 

 contained in some foods, such as the cereal grains, for the moisture and 

 warmth of the paunch are favorable to enzyme action in general. 



41. The small intestine. When received into the small intestine, the 

 partially digested food is a semi-liquid mass. As yet, the fats have 

 not been digested appreciably, and the digestion of the proteins and 

 carbohydrates is far from complete. Here the work of digestion proceeds 

 even more vigorously than in the stoma-ch, all classes of nutrients being 



