THE DIGESTIVE SECRETIONS 917 



the impulses apportioned to the submaxillary gland it may rightly be 

 concluded that the fibers innervating this organ pass directly through 

 the aforesaid ganglion without forming new connections. 



Facts Disproving the Filtration Theory. While it cannot be denied 

 that filtration plays an important part in the secretion of saliva, we are 

 not warranted in believing that it is the only factor mediating this 

 process. It has previously been noted that the excitation of the chorda 

 tympani gives rise to a vaso-dilatation and a copious flow of a very 

 watery type of saliva, while the stimulation of the cervical sympathetic 

 nerve evokes a vaso-constriction and a scanty flow of a very viscous 

 saliva. While these changes may at first be thought to favor filtration, 

 they cannot be interpreted in this way if contrasted with such facts 

 as the following: 



(a) We have noted above that the formation of saliva is not wholly 

 dependent upon the blood-supply, but is more closely related to the 

 influences of the nervous system. 



(6) It has also been pointed out that this secretion may be either 

 increased or decreased by drugs without altering the pressure existing 

 in the capillaries of the gland from which the material is taken. 1 



(c) Inasmuch as the saliva contains bodies, such as mucin and ptya- 

 lin, which are not present in the body-fluids, the secreting cells must 

 possess the specific power of forming them. Carlson states that saliva 

 also contains a diastase which is present here in smaller amounts than 

 in the blood. 



(d) It has been shown that saliva may also be secreted by the sub- 

 maxillary glands after its artery has been ligated, and that 



(e) The normal gland may be made to secrete against a higher pres- 

 sure than the capillary pressure. 



Under ordinary conditions, the salivary cells derive their secre- 

 tory material from the blood-capillaries in which the pressure does not 

 rise above 40 to 60 mm. Hg. They then discharge it into the salivary 

 duct in which the pressure approximates zero. This arrangement 

 favors filtration. It can be shown, however, that these cells are also 

 able to secrete against a pressure which not only exceeds the capillary 

 pressure, but also that prevailing in the carotid artery. Thus, if 

 Wharton's duct is connected with a mercury manometer, and the 

 chorda tympani is stimulated repeatedly at intervals, it will be found 

 that the mercury continues to rise until it eventually indicates a pres- 

 sure at least twice as high as that existing in the capillaries of the sub- 

 maxillary gland. In fact, Hill and Flack 2 have succeeded in obtaining a 

 salivary pressure of 240 mm. Hg against a blood-pressure of 130 mm. Hg. 

 Now, if filtration were the only factor concerned in the formation of 

 saliva, a relationship of this kind could not be successfully established. 

 The same conclusion may be derived from the fact that even the 



1 Sarmus, Zeitschr. fur Biologic, Iviii, 1912, 185. 



2 Proc. R. Soc. London, Ixxxv, 1912. 



