THE SALIVARY GLANDS 257 



position products found in the expired air. However, the bile passes into the 

 digestive canal only after the ingestion of food; in the meantime it is being 

 stored in the gall bladder. 



In 1868 Heidenhain published the important observation that after long- 

 continued activity the submaxillary gland exhibits morphological changes, 

 and some years later he ascertained that the same is true of the parotid and 

 of the fundus glands of the stomach. Investigation in this direction was 

 extended by several other authors, and it has been proved by their work that 

 while the gland is resting i. e., is not pouring out secretions a substance 

 is being laid down within it in the form of small granules, which to a greater 

 or less extent disappears during the activity of the gland. This substance 

 must be regarded as the source of the specific constituents of the glandular 

 secretions. 



Heat is generated by the glands in the act of secretion. Cl. Bernard 

 (1856) found the temperature of the hepatic blood constantly higher than 

 that of the portal blood. At the time of active secretion of bile the difference 

 rose to 0.7-0.9 C. The following year Ludwig and Spiess observed that 

 the temperature of the submaxillary saliva may be more than 1 C. higher 

 than that of the blood in the carotid of the same side. The increase in oxygen 

 consumption and of carbon-dioxide production indicate a highly active metab- 

 olism going on in a working gland; both are three to four times as great in 

 a strongly active condition of the submaxillary as in a resting condition 

 (Barcroft). 



Attention has already been directed to the electric phenomena of glands 

 (page 48). Bayliss and Bradford report that on stimulation of the cerebral 

 secretory nerves of the dog, a strong electric variation is produced both in the 

 submaxillary and in the parotid, since the surface of the gland becomes negative 

 to the hilus. Stimulation of the sympathetic produced an opposite variation 

 the surface becoming positive to the hilus. Moreover, they showed that these 

 electrical variations are not due either to alterations of the blood flow or to the 

 flow of the secretion through the duct. On the basis of these and other observa- 

 tions, the authors conclude that the negativity of the surface toward the hilus 

 is the result of a passage of fluid through the wall of the acini or is the result 

 of changes in the gland cells set up by stimulation, which precede the passage of 

 the fluid. The positivity of the surface would be the expression of those changes 

 in the gland cells by which the organic constituents of the secretion are formed. 



2. THE SALIVARY GLANDS 

 A. SECRETORY NERVES 



The salivary glands receive their nerves by two different pathways, namely 

 the cerebral and the sympathetic. The former were demonstrated by Ludwig as 

 mentioned on page 256; while the discovery that the sympathetic can cause 

 secretion of saliva we owe to Eckhard. 



In the dog the cerebral nerves to the submaxillary and sublingual glands 

 proceed from the facial nerve through the chorda tympani to the lingual branch 

 of the trigeminal, and from this along the ducts to the gland. The cerebral sup- 

 ply to the parotid of the dog springs from the glossopharyngeal and reaches the 



