144 MANUAL OF PHYSIOLOGY. 



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 second- 

 ary 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 sympathetic 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 

 the same animal. So far as regards the blood vessels, then, the 

 chorda is directly opposed to the sympathetic. To explain this 

 antagonism we may either assume the existence of local nerve 

 centres governing the muscular coats of the arterioles, and sup- 

 pose that the sympathetic stimulates and the chorda inhibits the 

 activity of these centres, or, what seems more simple, in the ab- 

 sence of anatomical evidence that such a centre exists, we may 

 attribute to the arterial muscle cells themselves an automatic 

 tonic power of contraction which can be increased by the sym- 

 pathetic and diminished by the chorda tympani. It is singular 



