372 PRINCIPLES OF GENERAL PHYSIOLOGY 



title page to his book (Fig. 100). These will be found in the description ; what 

 concerns us here is the dog in the foreground with salivary and pancreatic fistulae. 



For the various operative procedures required, the articles by Pavlov (1902) 

 and by London (1910) may be consulted. 



Saliva. This is the first fluid met with and it is produced even before the 

 food enters the mouth. This " psychical secretion " is caused by reflexes through 

 sight, smell, and so on ; the mouth "waters." The taste of the food in the mouth 

 causes renewed secretion. 



The Gastric Juice. When the food enters the stomach, it finds that gust ric 

 juice has already been secreted. This first secretion is psychical and depend^ 

 greatly on the appetite with which the eating of food is approached. It is 

 produced before food actually enters the mouth and, in fact, the mere presence of 

 food in the mouth, without appetite, does not excite secretion. When, therefore, 

 Macbeth wishes for his guests that "good digestion" may "wait on appetite," 

 he is merely expressing a physiological fact. 



If solid food is introduced directly into the stomach through an opening, a 

 gastric fistula, unknown to the dog under experiment, no secretion is produced for 

 an hour or more. Mechanical stimulation of the mucous membrane of the 

 stomach is also ineffective. 



The efferent nerve through which the glands of the stomach are excited is 

 the vagus. 



Certain chemical substances introduced into the stomach produce a secretion. 

 According to Edkins, as mentioned already, a hormone, analogous to the pan- 

 creatic secretin, is produced from the mucous membrane of the pyloric portion 

 and carried in the blood to excite the glands of the fundus. The experiments of 

 Pavlov in connection with the chemical mechanism were performed mainly on 

 dogs provided with a miniature stomach, separated from the main one by an 

 ingenious operation, which is a greatly improved form of a similar one done by 

 Heidenhain (see p. 13 of Pavlov's book). This miniature stomach was found 

 to serve as a sample or indicator of all that proceeded in the main stomach. 



It was found that meat juice or Liebig's extract caused secretion ; but no 

 result was obtained from raw egg white, nor from starch nor fat. That the 

 mechanism is a chemical one and not nervous is shown by the fact that Liebig's 

 extract and similar substances are effective after the vagus nerves have been 

 divided. 



The Pancreas. The mode of excitation of the pancreas was described in the 

 previous chapter. We see that the acid gastric contents, when they arrive in 

 the duodenum, give rise to the production of secretin, which excites the pancreas. 

 Whether the vagus takes any part in the normal process we have seen to be 

 doubtful. If it does so, there is a possibility of "psychical" secretion from 

 appetite, in addition to the effect of escape of the acid " psychical " gastric juice 

 passing into the duodenum. 



The Bile is another important secretion poured into the intestine. Its function 

 will be discussed presently. We have seen that the same acid extract of duodenum 

 which excites the pancreas also causes the secretion of bile, so that the acid 

 contents of the stomach when they arrive in the duodenum cause also a secretion 

 of bile. We have no evidence of a nervous control over the liver, with the 

 exception of vaso constrictor nerves to the branches of the portal vein. 



Snccus Enlericus, as already mentioned, appears to be excited, in any 

 particular part of the intestine, by the presence of pancreatic juice in the parts 

 preceding this one. 



THE CHANGES IN THE FOOD 



We may now proceed to describe the changes which the food undergoes in the 

 several parts of the alimentary canal. 



Since most food is taken in the form of more or less solid masses, a means of 

 disintegrating it is clearly of advantage for the ready access of enzymes Most 

 animals possess some means of doing this. Masticating apparatus, such as teeth 



