ACTION OF NERVES ON THE SECRETION OF SALIVA. 287 



occur, as when the origin of the seventh nerve itself is stimulated. The 

 watery saliva is called chorda saliva.] 



Two functionally different kinds of nerve-fibres occur in the facial 

 nerve (1) True secretory fibres, (2) Vasodilator fibres. The increased 

 amount of secretion is not due simply to the increased blood supply. 



II. Stimulation of the sympathetic nerve causes a scanty amount 

 of a very thick, sticky, mucous secretion (Eckhard), in which the 

 specific salivary constituents, mucin, and the salivary corpuscles are 

 very abundant. The specific gravity of the saliva is raised from 1,007 

 to 1,010. Simultaneously the blood-vessels become contracted, so that 

 the blood flows more slowly from the veins, and has a dark bluish 

 colour. 



The sympathetic also contains two kinds of nerve-fibres (1) True 

 secretory fibres, and (2) vaso-constrictor fibres. 



Relation to Stimulus. On stimulating the cerebral nerves, at first with a weak 

 and gradually with a stronger stimulus, there is a gradual development of the 

 secretion in which the solid constituents occasionally the organic are increased 

 (Heidenhain). If a strong stimulus be applied for a long time, the secretion 

 diminishes, becomes watery, and is poor in specific constituents, especially in the 

 organic elements, which are more affected than the inorganic (C. Ludwig and 

 Becher). After prolonged stimulation of the sympathetic, the secretion resembles the 

 chorda saliva. It would seem, therefore, that the chorda and sympathetic saliva 

 are not specifically distinct, but vary only in degree. On continuing the stimula- 

 tion of the nerves up to a certain maximal limit, the rapidity of secretion becomes 

 greater, and the percentage of salts also increases to a certain maximum, and this 

 independently of the former condition of the glands. The percentage of organic 

 constituents also depends on the strength of the nervous stimulation, but not on this 

 alone, as it is essentially contingent upon the condition of the gland before the 

 secretion took place, and it also depends upon the duration and intensity of the 

 previous secretory activity. Very strong stimulation of the gland leaves an 

 " after-effect " which predisposes it to give off organic constituents into the 

 secretion (Heidenhain). 



Relation to Blood Supply. The secretion of saliva is not simply the 

 result of the amount of blood in the glands ; that there is a factor 

 independent of the changes in the state of the vessels is shown by the 

 following facts : 



(1.) The secretory activity of the glands when their nerves are stimulated con- 

 tinues for some time after the blood-vessels of the gland have been ligatured 

 (Ludwig, Czermack). [If the head of a rabbit be cut off, stimulation of the seventh 

 nerve, above where the chorda leaves it, causes a flow of saliva which cannot be 

 accounted for on the supposition that the saliva already present in the salivary 

 glands is forced out of them. Thus we may have secretion without a blood-stream. 

 The saliva is really secreted from the lymph present in lymph-spaces of the gland 

 (Ludwig)]. 



(2.) Atropin and Daturin extinguish the activity of the secretory fibres in the 

 chorda tympani, but do not affect the vaso-dilator fibres (Heidenhain). The 

 same results occur after the injection of acids and alkalies into the excretory 

 duct (Gianuzzi). 



