756 PHYSIOLOGY OF DIGESTION AND SECRETION. 



to the pressure and composition of the secretion which will be referred 

 to presently. It was afterward shown that the salivary glands receive 

 a double nerve supply, in part by way of the cervical sympathetic 

 and in part through cerebral nerves. It was discovered also that 

 not only are secretory fibers carried to the glands by these paths, 

 but that vasomotor fibers are contained in the same nerves, 

 and the arrangement of these latter fibers is such that the cerebral 

 nerves contain vasodilator fibers that cause a dilatation of the small 

 arteries in the glands and an accelerated blood-flow, while the sym- 

 pathetic carries vasoconstrictor fibers whose stimulation causes a 

 constriction of the small arteries and a diminished blood-flow. The 

 effect of stimulating these two sets of fibers is found to vary somewhat 

 in different animals. For purposes of description we may confine 

 ourselves to the effects observed on dogs, since much of our funda- 

 mental knowledge upon the subject is derived from Heidenhain's* 

 experiments upon this animal. If the chorda tympani nerve is 

 stimulated by weak induction shocks, the gland begins to secrete 

 promptly, and the secretion, by proper regulation of the stimulation, 

 may be kept up for hours. The secretion thus obtained is thin and 

 watery, flows freely, is abundant in amount, and contains not more 

 than 1 or 2 per cent, of total solids. ' At the same time there is an 

 increased flow of blood through the gland. The whole gland takes 

 on a redder hue, the veins are distended, and if cut the blood that 

 flows from them is of a redder color than in the resting gland, and 

 may show a distinct pulse all of which points to a dilatation of the 

 small arteries. If now the sympathetic fibers are stimulated, quite 

 different results are obtained. The secretion is relatively small in 

 amount, flows slowly, is thick and turbid, and may contain as much 

 as 6 per cent, of total solids. At the same time the gland becomes 

 pale, and if the veins be cut the flow from them is slower than in 

 the resting gland, thus indicating that a vasoconstriction has 

 occurred. 



The increased vascular supply to the gland accompanying the 

 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 



* "Pfliiger's Archiv fur die gesammte Physiologic," 17, 1, 1878; also 

 in Hermann's "Handbuch der Physiologic," 1883, vol. v, part i. 



