752 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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 

 gland which had previously been secreting actively. With regard 

 to the organic constituents the precise result obtained depend? on 

 the condition of the gland. If previous to the stimulation the gland 

 was in a resting condition and unfatigued, then increased strength 

 of stimulation is followed at first by a rise in the percentage of organic 

 constituents, and this rise in the beginning is more marked than in 

 the case of the salts. But with continued stimulation the increase 

 in organic material soon ceases, and finally the amount begins actually 

 to diminish, and may fall to a low point in spite of the stronger 

 stimulation. On the other hand, if the gland at the beginning of the 



