490 LECTURE XXI. 



when we find not only that the amount of saliva is regulated according to 

 the requirements, but likewise its composition. At one time it contains 

 considerable mucin, at another time relatively little, according to the con- 

 ditions. Apparently the irritation, whether it be chemical, thermal, or 

 mechanical, stimulates the apparatus at the end of the afferent nerves 

 in the mucous membrane of the mouth. From here the impulses are 

 transmitted to the central organ of the nervous system, and now by 

 means of the efferent nerves the individual salivary glands are called 

 into action. We speak of such a process in general as a reflex action. 



It is of chief interest to us that it is possible by various outward effects 

 to influence the activity of the glands in such a way that the composition 

 of the saliva secreted is adjusted to the prevailing conditions. At 

 present we can only imagine how this adjustment may be effected. 

 The saliva itself always contains besides inorganic salts and water a certain 

 amount of organic material, as we have seen. We also repeat that 

 the saliva contains a ferment capable of hydrolyzing starch, the diastase. 

 It is remarkable too that it always contains small amounts of alkali 

 thiocyanate. We are wholly in the dark concerning the formation of this 

 last compound, or as regards its use. 



In this connection it seems fitting to mention some observations con- 

 cerning secretions in the mouths of certain invertebrates. They are of 

 particular value because they make it easier for us to understand the most 

 important work of the gland-cells. Here we meet with the preparation 

 of strong acids by means of cell activity from material which could not 

 have contained the acid already formed. Long ago Troschel * noted, in 

 examining a kind of snail, Dolium galea, that the animal squirted 

 from its mouth a stream of liquid clear as water. The liquid showed a 

 strongly acid reaction, and caused effervescence on coming in contact with 

 the limestone lying on the ground. The secretion was produced from two 

 large gland-like organs lying near the stomach. The ducts from it ascend 

 on each side of the gullet and empty into the mouth. The secretion con- 

 tains sulphuric acid, and, in fact, as much as 4.1 per cent of the free acid is 

 present. Besides this, there is 0.4 to 0.6 per cent of hydrochloric acid. 

 Other varieties of snails similarly produce acids. Some of these " acid 

 snails " have been quite recently studied by Fr. N. Schulz. 2 He followed 

 particularly closely the acid production on the part of the naked snail, 

 Pleurobranchia Meckelii of the order Opisthdbranchi. On being touched, 

 the snail rolls itself up. If the animal is squeezed a little, its external 

 surface after a short time becomes covered with a slimy secretion of acid 

 reaction. This comes from glands which are very numerous in the skin. 



1 Poggendorf J s Ann. 93, 614 (1854) ; J. pr. Chem. 63, 170 (1854) ; see also de Luca and 

 Panceri, Compt. rend. 65, 577 and 712 (1867). 



2 Z. allgem. Physiol. 6, 206 (1905). 



