DIGESTION IN THE MOUTH 376 



stations are shown by painting the ganglia with nicotine ; this 

 paralyzes the synapses or end terminations of the preganglioaic 

 fibres with the ganglion cells. 



There is some discussion as to the manner in which the effector 

 nerve acts. According to one view, there are two kinds of fibres in 

 the nerve trophic fibres, which cause changes in the granular material, 

 preparing it for its discharge from the cell; and the secretory fibres, 

 which bring about this discharge. Upon this view, the sympathetic 

 nerve is mainly trophic, the cranial (chorda) nerve is mainly secretory. 



It is more generally held, however, that one kind of fibre causes 

 all the changes attendant on secretion, the difference between chorda 

 and sympathetic saliva being due to the difference in blood-supply. 

 With the chorda stimulation there is a great vaso -dilatation, with 

 sympathetic stimulation there is marked constriction of the small 

 arteries supplying the gland. Vaso-dilatation is probably also pro- 

 duced locally by the products of mstabDlism of the gland. 



The action of the nervous mechanism appears to be more marked 

 at som3 times than others. It is more marked in a state of hunger than 

 in a state of satiety. It may possibly be that tie products of digestion 

 or the saliva itself b3com3 absorbed, and affect the excitability of the 

 nervous mechanism or the cells of the gland. 



A day or two after the chorda tympani nerve has b33n divided 

 there begins a slow continuous secretion of saliva, known as 

 paralytic S)cretion which lasts from five to six weeks. As the 

 nerve itself degenerates in thres to five days, the se3retion is 

 obviously due to some loeal apparatus in the gland. If the sympathetic 

 nerve bs cut at the same time, the 'secretion is diminished or stopped. 

 No paralytic secretion follows secretion of the sympathetic nerve by 

 itself; this produces little or no observable effest. The same is true 

 in regard to excision of the superior cervical ganglion. 



The characteristic saliva of any one gland may be obtained by 

 placing a small glass cannula in the main duct, and stimulating the 

 effector nerve. The submaxillary gland is the one which lends itself 

 most readily to this experiment. By such means the true secretory 

 nature of the saliva can bs demonstrated. If the cannula placed in 

 the submaxillary duct be connected to a manometer, and the pressure 

 which is produced in the duct during stimulation of the chorda tympani 

 be measured, it may be found to rise to twice the height of the pressure of 

 the arterial blood supplying the gland (Fig. 193). The salivary secretion 

 is therefore a true secretion. It is not merely a passage through into 

 the salivary ducts as the result of pure mechanical' processes, such as 

 filtration, diffusion, or osmosis. Even with so great a pressure, the 

 blood still flows through the salivary glands. This is because the 

 blood-capillaries of the gland are protected from oaclusion by the 

 basement membranes which enclose the alveoli of the gland. These 

 membranes, strengthened by connective tissue, limit the expansion 

 of the alveoli, just as the leather case of a football limits the expansion 

 of the bladder within. A certain amount of expansion is allowed, 

 however enough to narrow the veins within the gland, and convert the 



