670 PHYSIOLOGY OF DIGESTION AND SECRETION. 



the sublingual gland is rich in mucin. The parotid saliva of man 

 seems to be particularly rich in ptyalin as compared with that of the 

 submaxillary, while the secretion of the latter and that of the sub- 

 lingual gland give a stronger alkaline reaction than the parotid saliva. 



The Secretory Nerves. The existence of secretory nerves to the 

 salivary glands was discovered by Ludwig in 1851. The discovery is 

 particularly interesting in that it marks the beginning of our knowl- 

 edge of this kind of nerve fiber. Ludwig found that stimulation of 

 the chorda tympani nerve causes a flow of saliva from the submaxil- 

 lary gland. He established also several important facts with regard 

 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 



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

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



