424 ANIMAL BIOCHEMISTRY 



through the abdomen into the stomach. This abnormality had 

 residted from a musket wound and permitted the introduction into 

 and removal of materials from the stomach. Similar fistulas have been 

 created in animals for studies of digestive processes. 



SALIVARY DIGESTION 



The first step in digestion by mammals is mastication. Teeth, 

 tongue, and lips cooperate in breaking the food into small particles 

 with a large surface area for subsequent attack by the digestive juices. 



The first of these juices with which the food comes in contact is 

 saliva secreted into the mouth by the salivary glands and the numer- 

 ous small buccal glands. There are three pairs of salivary glands, 

 parotid, sublingual, and submaxillary glands. When food is eaten 

 these glands are stimulated, and the flow of saliva increases. Increased 

 salivation may also be induced by placing objects in the mouth, by 

 the sight or thought of food, by conditioned reflexes associated with 

 feeding, by odors and flavors, and by stimulating certain connecting 

 nerves. Human beings secrete 1 to 1.5 liters of saliva daily and horses 

 and cattle perhaps 40 and 50 liters, respectively. The amounts appear 

 to vary with the nature of the food, its moisture content, and environ- 

 mental and psychic factors. To some extent the composition and 

 hence the consistency vary in a similar way. 



Saliva is, of course, primarily water but contains a variety of 

 inorganic ions like sodium, potassium, calcium, magnesium, chloride, 

 carbonate, phosphate, and sulfate, and has a jjH in the range of 6.4 

 to 7.0. On standing the pH tends to rise as a result of loss of carbon 

 dioxide. In addition to the inorganics, an organic fraction consists 

 of several enzymes, small amounts of a variety of soluble organic com- 

 pounds, and mucin, a glycoprotein which forms viscous solutions and 

 serves as a lubricant. Besides water mucin is the most abundant com- 

 ponent in saliva. 



Of the enzymes present, only ptyalin (salivary amylase) is known 

 to be important, although even it does not occur in the salivas of the 

 carnivores and herbivores. Ptyalin is produced in quantity by man, 

 SAvinc, and rodents and partially hydrolyzes starch, producing maltose 

 and dextrins of various sizes. Since maltase also is secreted in traces, 

 some maltose may be split to glucose but the total amount is probably 

 quite small, partly because of the low enzyme concentration. Further- 

 more, the food spends such a short time in the mouth that there is 



