154 Comparative Animal Physiology 



enzymes are secreted characterizes vertebrates. Mammals show a definite 

 sequence of secretion in both protein and carbohydrate digestion; fish show 

 sequential secretion for protein only. 



During the course of digestion, food is subjected to successive changes in 

 the medium. This is true even in an intracellular digestive cycle in which, as 

 in Protozoa, the food vacuole may change in acidity, water content, osmotic 

 concentration, and enzyme content. In extracellular digestion, the various 

 glands secrete a variety of acids, salts, and other substances besides enzymes. 

 In most animals the hydrogen ion concentration of the digestion mixture 

 changes during the course of digestion or during passage along the alimentary 

 tract. Specific enzymes function only over a limited pH range and are in- 

 activated by extremes of acidity or alkalinity. However, the pf^ optimum of an 

 enzyme is not necessarily the same as the pH in the digestive organ in which 

 the enzyme normally functions. 



Table 25 gives some selected values of the pH of digestive fluid or of the 

 contents of different digestive chambers in representatives of several animal 

 groups. A more detailed table is given by Kruger. ^■* Food in a digestive 

 organ affects the acidity, and the acidity of secreted fluids differs in resting 

 and in actively digesting states. A general pattern of acidity changes is indi- 

 cated, however, in Table 25. 



In mammals food passes through a series of changes in pH. The mouth is 

 nearly neutral in reaction. In man under basal conditions parotid saliva has 

 an average pH of 5.8, and mandibular (mixed) saliva a pH of 6.4, although the 

 pH may rise as much as two units in response to chemical and other stimuli. ^^"" 

 The saliva of other mammals is reported to be alkaline, although methods of 

 collection have not always excluded loss of CO2 to the air. Saliva of the cow is 

 more alkaline (pH 8.8) than the regurgitate from the rumen (pH 7.9). '^^ 

 The copious liquid and the mucin content of mammalian saliva make it a good 

 food lubricant. Man secretes 0.5 to 1 liter of saliva per day, and animals on 

 dry diets secrete much more; a horse may secrete 40 liters, and a cow 60 liters 

 daily; ^"'^ each parotid gland of a sheep secretes 930 to 1840 ml./24 hr. ^-- 



In the mammalian stomach the parietal cells secrete HCl to the extent of 

 0.4 to 0.5 per cent (pH 0.91). When diluted with food, the contents of the 

 stomach range from pH 1.5 to 2.5 in normal humans. Part of the acid is free, 

 more is combined. The acid of the stomach is bactericidal, kills cells of food 

 which may be alive when swallowed, and may aid in decalcification of food.'^ 

 Stomach acid also activates the principal gastric enzyme, pepsin, and provides 

 an optimal pH for its action. Monotremes lack stomach glands. In ruminants 

 the storage portions of the stomach are alkaline, and only the true stomach or 

 abomasum is acid. In young chickens the gizzard is more acid than it is in 

 adult hens. The stomach contents in frogs are nearly as acid as those in mam- 

 mals. In the bony fish which have a stomach the gastric pH is not as low as it 

 is in mammals, but elasmobranchs have extremely acid gastric contents. Stom- 

 achless fish swallow food directly into the alkaline intestine (Fig. 29, D). 



Among invertebrate animals the relation of acidity of the digestive tract to 

 diet and to the presence or absence of food is even more marked than it is 

 among vertebrates. In general the anterior regions of the digestive system are 

 nearly neutral or weakly acid (Table 25). In molluscs and some insects they 

 are acid, but in the earthworm the region of the pharynx is alkaline. The crop 



