DIGESTION IN ANIMALS. 597 



the upper part of the intestines, which may be pancreatic. Similar rudimen- 

 tary parts exist in the Rotifera, amongst the Annuloida. In cases hi which 

 such tubuli are not present, gland cells are sometimes found in patches upon 

 the lining membrane of the intestine. Sometimes even these are not distin- 

 guishable. 



Intestinal Glands. In all the Yertebrata, besides structures resembling the 

 closed sacs of the solitary and agminated glands, the intestinal tubuli or crypts 

 of Lieberklihn exist. In Mammalia and Birds, and probably in Reptiles, 

 Amphibia, and the higher Fishes, racemose mucous glands are found in the 

 duodenum. 



The tubular or saccular appendages, the short cseca, or the minute patches 

 of glandular epithelial cells, distinguished by their color and contents, which 

 are found in the Molluscous, Annulose, and lower allied forms, are probably 

 not representatives of the intestinal tubuli in the higher animals, but rather 

 of the liver and pancreas. Indeed, the intestinal canal is itself so minute in 

 many of these lower animals, that its lining membrane is almost of necessity 

 simple, smooth, and covered throughout with a delicate epithelium only. 



The Chemical Processes of Digestion in Animals. 



In studying the action of the digestive fluids, physiologists have employed 

 not only the human secretions, but also, and sometimes exclusively, those col- 

 lected from fistulas in animals, and likewise artificial fluids made by macerating 

 the glands in water. The properties of these several secretions having been 

 established experimentally, in regard to certain vertebrate animals, it is rea- 

 sonable to conclude that, wherever these particular glands exist, the respective 

 secretions possess similar, if not identical, properties. 



The saliva is most abundant in herbivorous and granivorous animals, in 

 which the quantity of food to be moistened is greater, and the special action 

 of this fluid on starchy matter is most required ; in carnivorous and insectiv- 

 orous creatures, this action is not necessary, and the secretion is less plentiful, 

 being used rather for purposes of lubrication ; or, as in Fishes, it may even 

 be wanting. Nevertheless, the saliva of the dog, taken from the mouth, con- 

 verts starch into sugar, though somewhat slowly. It is said, however, that 

 the secretions of the parotid and submaxillary glands in the dog, and even 01 

 the parotid only, in the horse, are by themselves, and unmixed with the 

 mucus of the mouth, incapable of effecting this transformation. 



In those Mollusca, Annulosa, and Annuloida, in which, as in Cephal- 

 opoda, Insecta, Myriapoda, and Rotifera, glands, called salivary, exist, the 

 exact properties of the secretion, and its chemical action on the food, have not 

 yet been determined ; but it is usual to regard those glands, whether tubular 

 or follicular, which open into the upper part of the alimentary canal, as sali- 

 vary glands. On similar grounds, gland structures opening into the stom- 

 ach are considered as gastric glands ; those connected with the upper part of 

 the intestine, as hepatic, or hepatic and pancreatic ; and, lastly, those empty- 

 ing themselves into the lower part of the intestine, as excretory, and proba- 

 bly renal. 



As albuminoid substances are essential to the formation of all animal tis- 

 sues, the gastric juice, which acts upon them, would appear to be likewise an 

 essential solvent in the digestive function of every animal. Hence, it is prob- 

 ably present in all animals, certainly in the lowest which possess a stomach, 

 and even t in the Coelenterata. When no distinct gastric tubuli exist, the 

 peptic agent is secreted by the cells of the lining membrane of the digestive 

 cavity. 



The stomachs of Fishes, after death, are often rapidly digested by their own 

 gastric juice ; whereas, this occurrence is much less frequent in Man and 

 Mammalia. This has been referred to the small difference of temperature 

 which takes place after death, in a fish, as compared with a warm-blooded 

 animal. The gastric juice of fishes habitually acts at a low temperature ; 

 whilst that of the warm-blooded animal operates at a much higher tempera- 

 ture. It is said that the gastric juice of fishes loses its peptic properties at 



