ACTION OF NERVES ON THE SECRETION OF SALIVA. 213 



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). A latent 

 period of 1 *2 sec. to 24 sec. may elapse between the nerve-stimulation and the beginning of the 

 secretion. 



[Langley has shown that in the cat the sympathetic saliva of the sub-maxillary gland is less 

 viscid than the chorda saliva.] 



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 continues for some 

 time after the blood-vessels of the gland have been ligatured. [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 the lymph-spaces of the gland (Lndwig).] 



(2) Atropin and daturin abolish 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). 



[Action of Atropin. The vascular dilatation and the increased flow of saliva, 

 due to the activity of the secretory cells, produced by stimulation of the chorda 

 tympani, although they occur simultaneously, do not stand in the relation of cause 

 and effect. We may cause vascular dilatation without an increased flow of saliva, 

 as already stated (2). If atropin be given to an animal, stimulation of the chorda 

 produces dilatation of the blood-vessels, but no secretion of saliva. Atropin 

 paralyses the secretory fibres, but not the vaso-dilator fibres (fig. 160). The 

 increased supply of blood, while not causing, yet favours the act of secretion, by 

 placing a larger amount of pabulum at the disposal of the secretory elements, the 

 cells.] 



(3) The pressure in the excretory duct of the salivary gland measured by 

 means of a manometer tied into it may be nearly twice as great as the pressure 

 within the arteries of the glands, or even in the carotid itself {Ludwig). The 

 pressure in Wharton's duct may reach 200 mm. Hg. 



[Secretory Pressure. The experiment described under (3) proves, in a definite 

 manner, that the passage of the water from the blood-vessels, or at least from the 

 lymph into the acini of the gland, cannot be due to the blood-pressure ; that, in 

 fact, it is not a mere process of filtration, such as occurs in the glomeruli of the 

 kidney. In the case of the salivary gland, where the pressure within the gland 

 may be double that of the arterial pressure, the water actually moves from the 

 lymph-spaces against very great resistance. We can only account for this result 

 by ascribing it to the secretory activity of the gland-cells themselves. Whether 

 the activities of the gland-cells, as suggested by Heidenhain, are governed directly 

 by two distinct kinds of nerve-fibres, a set of solid-secreting fibres, and a set of 

 water-secreting fibres, remains to be proved.] 



(4) Just as in the case of muscles and nerves, the salivary glands become fatigued or 

 exhausted after prolonged action. This result may also be brought about by injecting acids or 

 alkalies into the duct, which shows that the secretory activity of the gland is independent of 

 the circulation {Gianuzzi). 



All these facts lead us to conclude that the nerves exercise a direct effect upon the secretory 

 cells, apart from their action on the blood-vessels. 



Extirpation of Salivary Glands. When the chorda tympani is extirpated on one side in 

 young dogs, the sub-maxillary gland on that side does not develop so much its weight is 50 

 per cent, less while the mucous cells and the '* crescents " are smaller than on the sound side 

 (Bufalini). 



During secretion, the temperature of the gland rises 1 *5 C. (Ludmg), and the 

 blood flowing from the veins is often warmer than the arterial blood. [The electro- 

 motive changes are referred to at p. 212.] 



