SECRETION. 163 



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, as briefly described 

 on p. 159. It was discovered also that not only are secretory fibres carried 

 to the glands by these paths, but that the vaso-motor fibres are contained in 

 the same nerves, and the arrangement of these latter fibres is such that the 

 cerebral nerves contain vaso-dilator fibres which cause a dilatation of the small 

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

 carries vaso-constrictor fibres whose stimulation causes a constriction of the 

 small arteries and a diminished blood-flow. The effect upon the secretion of 

 stimulation of these two sets of fibres is found to vary somewhat in different 

 animals. For purposes of description we may confine ourselves to the effects 

 observed on dogs, since most of our fundamental knowledge upon the subject 

 is derived from Heidenhain's l 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 stimuli, 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 dilata- 

 tion of the small arteries. If now the sympathetic fibres 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 vaso-constriction 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 vaso-motor 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 erro- 

 neous and that definite secretory fibres 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 will still give a secretion for a short time. (3) If atropin is 



1 Pflilger's Archiv fur die gesammte Physiologic, 1878, Bd. xvii. p. 1 ; also in Hermann's Haml- 

 huch dr Physioloffie, 1883, Bd. v. Th. 1. 



