DIGESTION 431 



by ruminants. In part this abundance of saliva contributes licjuid 

 needed for active microbial growth and lermentation. The micro- 

 organisms secure the required energy and carbon sources by attacking 

 the plant carbohydrates. Likewise they make all the intermediates, 

 vitamins, and coenzymes needed for their growth. 



However, the nitrogen supply sometimes limits the microbial ac- 

 tivity which must idtimately depend upon the protein syntheses. 

 Various forms of nitrogen have been incorporated into diets for rumi- 

 nants in an effort to increase the utilization of cellulose and conse- 

 quently the efficiency of grazing animals. Reduced nitrogen of almost 

 any non-toxic type is effective, and it is now economically feasible to 

 make urea supplements to cattle rations. 



Newborn ruminants have no rumen flora and derive no benefit 

 from cellulose and the like until a suitable flora develops. 1 his takes 

 from 4 to 6 weeks in the calf, probably depending on accidents of in- 

 gestion of suitable organisms. Thereafter the rumen performs its func- 

 tion except in abnormal cases, the most common being bloat or tym- 

 pany. In this state the rumen becomes distended, perhaps with fatal 

 residts, because of excessive gas production, failure of the belching 

 mechanism, or formation of froth in the rumen. The most common 

 cause is associated by some unknown mechanism with extensive con- 

 simiption of green legumes. Several treatments are available. 



Fowl 



The digestive tracts of domestic fowl differ somewhat from those 

 of mammals and thereby warrant separate consideration. The esopha- 

 gus (gullet) of chickens differs by being divided into two parts known 

 as the first and second portions. These are separated (see Figure 17-.S) 

 by an enlargement of the esophagus known as the crop, which func- 

 tions as a storage organ during the hours of feeding. It should be 

 pointed out that no mastication takes place, since birds do not 

 possess teeth. 



From the crop the food passes through the second portion of the 

 esophagus to the proventriculus or glandidar stomach, which is a 

 small expansion of the esophagus, measuring (in the hen) about 

 li/, i"- '" length and 1 in. in diameter. The proventriculus finictions 

 primarily as a secretory organ, furnishing gastric juice, but it differs 

 from the true stomach of other animals in that the food does not 

 remain long enough for gastric digestion to take place. From the 

 proventriculus, the food, moistened with gastric juice, passes to the 

 gizzard. This large muscular organ, with the aid of grit, grinds 

 the food material to a fine state of division. During the grinding 



