692 PHYSIOLOGY OF DIGESTION AND SECRETION. 



abundant flow of "chorda saliva" and the diminished flow of blood 

 during the scanty secretion of " sympathetic saliva " suggest naturally 

 the idea that the whole process of secretion may be, at bottom, a 

 vasomotor phenomenon, the amount of secretion depending only on 

 the quantity and pressure of the blood flowing through the gland. 

 It has been shown conclusively that this idea is erroneous and that 

 definite secretory fibers exist. The following facts may be quoted 

 in support of this statement: (1) Ludwig showed that if a mercury 

 manometer is connected with the duct of the submaxillary gland and 

 the chorda is then stimulated for a certain time, the pressure in the 

 duct may become greater than the blood-pressure in the gland. 

 This fact shows that the secretion is not derived entirely by processes 

 of filtration from the blood. (2) If the blood-flow be shut off 

 completely from the gland, stimulation of the chorda still gives a 

 secretion for a short time. (3) If atropin is injected into the gland, 

 stimulation of the chorda causes vascular dilatation, but no 

 secretion. This may be explained by supposing that the atropin 

 paralyzes the secretory, but not the dilator fibers. (4) Hydro- 

 chlorate of quinin injected into the gland causes vascular dilatation, 

 but no secretion. In this case the secretory fibers are still irritable, 

 since stimulation of the chorda gives the usual secretion. 



A still more marked difference between the effect of stimulation 

 of the cerebral and the sympathetic fibers may be observed in the 

 case of the parotid gland in the dog. Stimulation of the cerebral 

 fibers, in any part of their course, gives an abundant, thin, and 

 watery saliva, poor in solid constituents. Stimulation of the sym- 

 pathetic fibers alone (provided the cerebral fibers have not been 

 stimulated shortly before and the tympanic nerve has been cut to 

 prevent a reflex effect) gives usually no perceptible secretion at all. 

 But in this last stimulation a marked effect is produced upon the 

 gland, in spite of the absence of a visible secretion. This is shown by 

 the fact that subsequent or simultaneous stimulation of the cerebral 

 fibers causes a secretion very unlike that given by the cerebral fibers 

 alone, in that it is very rich indeed in organic constituents. The 

 amount of organic matter in the secretion may be tenfold that of the 

 saliva obtained by stimulation of the cerebral fibers alone. 



Relation of the Composition of the Secretion to the Strength of Stimu- 

 lation. If the stimulus to the chorda is gradually increased in 

 strength, care being taken not to fatigue the gland, the chemical 

 composition of the secretion is found to change with regard to the 

 relative amounts of the water, the salts, and the organic material. 

 The water and the salts increase in amount with the increased 

 strength of stimulus up to a certain maximal limit, which for the 

 salts is about 0.77 per cent. It is important to observe that this 

 effect may be obtained from a perfectly fresh gland as well as from a 



