42 PHYSIOLOGY FOR DENTAL STUDENTS. 



revealed by histological examination of the gland, there is only 

 a slight flow of saliva from the duct because of the concomitant 

 curtailment of the blood supply. In so far as actual secretion of 

 saliva is concerned, the net result of stimulation of either nerve 

 is therefore dependent upon whether dilatation or constriction 

 of the blood vessels of the gland occurs, and this might lead us 

 to conclude that the secretion is secondary to changes in the 

 blood supply; in other words, that it is unnecessary to assume 

 the independent existence of specific secretory nerve impulses. 

 That such secretory fibers do exist, however, is established by 

 many facts. Two of these are : ( 1 ) The vessels still dilate but 

 no secretion occurs after a certain amount of atropin has been 

 allowe'd to act on the gland. This alkaloid paralyzes the secre- 

 tory nerve fibers, but has no action on those concerned in vaso- 

 dilation. (2) If the secretions were merely the result of in- 

 creased blood supply, in other words, were a filtrate from the 

 blood, the pressure in the duct would at all times be less than 

 that in the blood vessels, but this is not the case, for during stim- 

 ulation of the cerebral nerves the duct pressure may rise far 

 above that of the blood vessels. 



But it must never be lost sight of that although both kinds of 

 fibers do exist, they are very closely associated in their action. 



The Reflex Nervous Control of Salivary Secretion. The 

 structural differences between the parotid and submaxillary 

 glands suggest that their functions may not be the same; that 

 their respective secretions must be required for different pur- 

 poses. To put this supposition to the test, it becomes necessary 

 to -adopt some means by which the conditions calling forth 

 the secretion of each gland may be separately studied. This can 

 be accomplished by a small surgical operation in which the ducts 

 are transplanted so as to discharge through fistulae in the cheek, 

 the secretion being easily collected, by allowing it to flow into a 

 funnel which is tied in place. 



In general, two distinct types of stimuli may call forth secretion 

 of one or other gland, namely: (1) direct stimulation of senxorii 

 nerve endings in the mouth, and (2) psychological stimuli in- 

 volving more or less of an association of ideas. 



