THE ANIMAL BODY DIGESTION METABOLISM 31 



In studying other causes which might produce the secretion, it was 

 found that no flow could be started by such mechanical stimulation as 

 passing a feather or a glass rod over the mucous membrane of the stom- 

 ach. Water caused a moderate flow of gastric juice, but when fat, egg 

 albumin, starch, or sugar was introduced with water no greater flow re- 

 sulted than with water alone. However, the juice of meat, which con- 

 tains simple nitrogenous compounds, called forth a marked flow. This 

 explains the continuance of the secretion after the psychic secretion 

 ceases. The gastric juice secreted as a result of the mental stimulus 

 digests some of the protein of the food, forming soluble nitrogenous 

 compounds, which in turn stimulate the glands to further secretion. 



The saliva secreted was thin and watery when sand or dry, powdered 

 biscuit was placed in the dog's mouth, and much more concentrated 

 when stones were introduced, which the dog could swallow without the 

 aid of a large amount of saliva. The amount of saliva and gastric juice 

 also depended on the nature of the food fed. Pawlow's work indicated 

 that the enzyme content of the digestive juices depended on the kind of 

 food, the glands being guided by a form of instinct, so that, for example, 

 the pancreatic juice would contain more trypsin when meat was fed than 

 when starch was supplied. However fascinating this idea is, after numer- 

 ous more recent investigations the consensus of opinion is now against 

 such an adaptation of the digestive juices to the nature of the food. 



It is indeed fortunate that the character of the digestive juices of an 

 animal is not changed with variations in the food consumed. Between 

 meal times the secretory cells are elaborating the enzymes which are to 

 be contained in the secretions that will be poured forth to digest the 

 next meal. If the cells formed only enzymes suited to the digestion of 

 the previous meal and the animal then consumed food of a different kind 

 at the following meal, the juices might be unsuited to its digestion. It is 

 therefore wise that no matter what food the animal consumes, the digest- 

 ive glands pour forth the enzymes needed for the digestion of all the 

 various nutrients. 



56. Palatability. The palatability of feeds is a factor of no small im- 

 portance in the feeding of stock. The experiments which have just been 

 reviewed show clearly that the mere sight or smell of well-liked food will 

 cause some flow of gastric juice. It is reasonable to believe, therefore, 

 that well-liked feeds are digested better than others which may be equally 

 nutritious but less palatable. 



Familiarity and habit are important factors concerned with the 

 palatability of feeds. When corn silage is first placed before cows, not 

 infrequently, after sniffing it, they will let it alone for a time. They 

 then usually begin nibbling at it, and later eat it with great relish. In 

 such cases food that at first seems unpalatable finally becomes palatable. 

 In his early experience the senior author was feeding two lots of fat- 

 tening steers, one on shelled corn and wheat bran, the other on ground 

 corn and wheat bran. After some weeks of successful feeding, the 



