THE REGULATION OF SALIVARY SECRETION. 187 



ical compounds from those brought them in the blood, and 

 secondly a power of selection in that they take out of the 

 blood and secrete into the salivary ducts only certain of 

 the constituents of the blood-plasma. How little the blood- 

 pressure by itself is of any importance in these processes of 

 secretion is indicated not only by the facts already cited, 

 that an increase in blood -pressure need not be followed by 

 an increased secretion and vice versa, but also by LUDWIG'S 

 classical observation. If one mercury manometer is tied 

 into WHARTON'S duct, while another is connected with the 

 carotid artery, and the chorda tympani nerve is stimulated 

 electrically in order to bring about a secretion of saliva, 

 the pressure registered in the salivary duct may be 100 to 

 200 mm. higher than that in the artery, "rlt is possible that 

 osmotic forces brought into play through a breaking down 

 of complex molecules into simpler ones may explain a part 

 of the phenomena observed, but.it seems much more prob- 

 able that the great pressures produced through the swelling 

 of colloids (such as mucin) in water are chiefly responsible. 

 Not only are colloids having a great affinity for water pro- 

 duced in the salivary glands, but histological evidence is 

 at hand to indicate that during secretion those portions of 

 the gland-cells lying farthest from the nucleus suffer the 

 greatest changes in size. Since the nucleus is intimately 

 connected with processes of intracellular oxidation, it is 

 conceivable that in those portions of the cell lying nearest 

 the lumen substances are formed which particularly favor 

 the imbibition of water by the colloids formed in the cells. 

 In this way the cells at first increase in size, and pres- 

 sures are produced which serve to squeeze certain por- 

 tions of the cell contents (the saliva) into the glandular 

 ducts. Finally, it is not impossible that substances capable 

 of swelling may be secreted into the smaller salivary duots 

 and that the pressures registered in the manometer con- 

 nected with the main salivary duct may be due, in part at 

 least, to the subsequent swelling cf these substances. 



