

NERVE-MECHANISM OF SALIVARY SECRETION. 135 



cury, while the blood pressure in the carotid artery of the animal 

 was only 112 m.m. (about 4| inches) mercury; that is to say, the 

 force by means of which secretion is driven outwards, is nearly 

 twice as great as the pressure in the bloodvessels in the gland. 

 The secretion of saliva cannot then be a question of mere filtra- 

 tion, for if the physical agency pressure alone were acting, the 

 saliva would be forced into the bloodvessels as soon as the pres- 

 sure in the duct exceeded that in the vessels. 



The force and rate with which the secretion is produced vary 

 with the strength of the stimulation. The flow of saliva steadily 

 increases within certain limits as the stimulus gets stronger. It 

 is not only the quantity of the secretion that depends on the 

 amount of nerve impulse, but also its quality ; that is to say, with 

 a fresh gland, not wearied by previous experiment, the amount 

 of solids in the saliva increases as the stimulus is increased, so 

 that not only the activity of the gland cells is under the control 

 of nerve influence, but the kind of work they perform is also 

 regulated by the intensity of nerve impulse they receive. 



It has been found that the increase in the blood flow is secon- 

 dary to the secretion called forth by stimulation of the chorda 

 tympani. This is shown by the fact that even when the blood 

 supply is cut off by any means (strong sympathethic stimulation, 

 ligature of the vessels, or even decapitation) an amount of saliva 

 can be made to flow from the gland which could not have been 

 stored up in its cells prior to the stimulation of this nerve. 



II. With regard to the influence exerted by the sympathetic 

 branches, the most obvious result of stimulation of these is a con- 

 traction in the arterioles, and a consequent diminution of the 

 amount of blood flowing through the gland. The glands look 

 pale, and the blood leaving them is intensely venous in character ; 

 the exact opposite, in fact, to the result obtained by stimulation 

 of the cerebro-spinal nerves. But the sympathetic has also an 

 effect on the gland cell, as it produces an increased flow of saliva. 

 In the dog the secretion of "sympathetic saliva" is only tempo- 

 rary and scanty, having high specific gravity, and being over- 

 loaded with the solids. In the cat and rabbit "sympathetic 

 saliva" is scanty, and not thicker than the "chorda saliva" of 



