ii EXTERNAL DIGESTIVE SECRETIONS 71 



excite much less. Strongly irritating substances, e.g. acids, salts, 

 etc., determine a copious flow of saliva, by which they are diluted 

 and their irritating action reduced. The saliva secreted under 

 these conditions is watery, and contains little mucin. 



Excitation of the secretory centres of the salivary glands may 

 occur not only from stimuli in direct contact with the mucous 

 membrane of the buccal cavity, but also from excitation at a 

 distance, by the action of various stimuli on the different sense 

 organs, e.g. nose, eye, ear. Since it is impossible to find any other 

 explanation for this kind of stimulation, Pawlow calls it a 

 psychical excitation. If, e.g., a hungry animal is shown a bit of 

 bread or other food, a secretion results, while there is absolutely 

 no response on showing it to another that has eaten to repletion. 

 If some food or other substance that provokes nausea is shown the 

 dog several times in succession, the reaction is lessened till it dies 

 out. Yet if a little of the nauseous substance which no longer 

 evokes secretion is placed in the mouth, the first response reappears, 

 and lasts for a certain time. 



The smell or other external sign of food is enough to determine 

 secretion. If, e.g., the hand, smelling of meat, is presented to the 

 dog, a flow of saliva is excited. If an acid has once been coloured 

 black, the sight of any black fluid will provoke secretion, assuming 

 of course that the black acid was introduced into the animal's 

 mouth on at least one occasion. 



According to Malloizel's observations (1902) on dogs with a 

 permanent fistula, the reflex secretion of saliva is specific for 

 different peripheral stimuli. Thus the saliva excited by the action 

 of salts, sulphate of quinine, or sand, is thin and contains less than 

 1 cgrm. of niucin in 6 c.c. of saliva : the saliva excited by raw meat, 

 on the contrary, is very viscid, containing 1-2 cgrm. of mucin in 

 1 c.c. of saliva ; that excited by sugar is between the two. Henri 

 and Malloizel further found that the diastatic activity of reflexly 

 excited saliva varies with different stimuli ; it is greater for meat 

 than for salts. 



The sight or smell of different substances, again, excites a 

 specific secretion of saliva. Section of the chorda tympani 

 abolishes every secretory reflex, while section of the sympathetic 

 has no effect. 



All these, like the preceding, are phenomena of reflex secretion, 

 in which the excitation travels along the afferent paths to the 

 centres, which then transmit it by the efferent secretory paths. 

 Certain experiments of Cl. Bernard, Eckhard, Loeb, Griitzner, 

 and Chlapowski show that the centres of salivary secretion are 

 localised in the bulb, probably at the origin of the facial and 

 glosso-pharyngeal nerves. In fact, when the bulb is separated 

 from the spinal cord by a cross-section, salivary secretion can no 

 longer be excited by the same means. On electrical excitation, or 



