368 A TEXTBOOK OF PHYSIOLOGY 



Generally, throughout both the vegetable and animal world, we 

 find foodstuffs are taken into the living cells in a state of simple solu- 

 tion, either already dissolved in water, or brought into solution by 

 enzymes and Avater secreted by the cells. From the protozoon which 

 engulfs its food up to the mammal is this true. True also is it of the 

 insectivorous plants, such as the sundew and pitcher-plant (the 

 former entraps insects with nets, or the latter with lethal wells of 

 water); of the yeast or bacterium; and of plants generally. 



The Mechanism of Secretion. Two methods of calling forth secre- 

 tion are employed: (1) The nervous reflex; (2) the chemical reflex, or 

 " hormone " mechanism. 



One or both of these mechanisms may be used to provide a juice. 

 More exact details are given when each juice is considered separately. 

 Nervous tissue has been elaborated for the especial purpose of 

 quick transference of messages from one part of the body to another. 

 The nervous mechanism is called into play when rapid secretion is 

 wanted. The nervous mechanism is therefore used for the supply 

 of the saliva and for the first flow of the gastric juice. While 

 enough fluid for the immediate demands of the body is provided by 

 the nervous mechanism, the chemical mechanism is present to insure 

 the presence of an adequate amount of digestive juices for the thorough 

 preparation of food and its complete digestion. 



For the liberation of the "hormone reflex," either (1) the prod- 

 ucts of the digestion brought about by " nervous "' flow, or (2) some 

 constituent of the juices so secreted, is concerned in calling forth this 

 ' ; chemical " flow of juice. For example, in the stomach we find that 

 the presence of dextrin and peptones that is, the products of a 

 salivary and gastric digestion respectively liberate from the pyloric 

 mucous membrane a body " gastrin " which is absorbed into the 

 blood, and, reaching the gastric glands, excites a further flow of gastric 

 juice. It may be also that there is something in the saliva able to 

 bring this mechanism into action, for it has been noticed that swallowed 

 saliva appears to have the power of evoking a flow of gastric jui- . 



The flow of pancreatic juice K also brought about by a " chemical " 

 re fl ex namely, by the liberation from the duodenal mucous mem- 

 brane of a body termed " secretin," w r hich passes in the blood to the 

 pancreas, and stimulates that organ to activity. There is evidence 

 that the pancreas may also be excited to secrete by impulses reaching 

 it through its nerves. This, however, does not appear to be the norm -a 1 

 mechanism. In the case of secretion excited by nervous mechanisms, 

 it is probable that the nervous excitation of the gland evokes a " hor- 

 mone " in the gland itself, which excites the secretion. For example, 

 an extract of resting salivary gland has no effect when injected into 

 the blood, but an extract of the same gland, after stimulation of th 

 chorda tympani nerve, is said to provoke secretion of saliva when 

 injected. 



The exact nature of the substance provoking the flow of succus 

 entericus is not well known. Undoubtedly, the acidity of the chyme 

 entering the duodenum plays a most important part as regards the 



