294 



Major W. B. Cannon. 



regions are now exposed to air, however, instead of being flushed by a flowing 

 stream, and consequently they tend to dry. The structural lining of these 

 parts probably renders them especially liable to desiccation in the presence of 

 dry air, for the mucosa of the mouth and also of the pharynx, below the level 

 of the floor of the nasal chambers, is composed of squamous epithelium Some 

 scattered mucous glands are present, but they are not capable of keeping the 

 surfaces satisfactorily wet, as any one can readily prove by breathing through 

 the mouth for only a few minutes. When air passes to and fro by way of 

 this watercourse, as in prolonged speaking or singing, and in smoking, it is 

 to be expected, therefore, that feeHngs of dryness and stickiness, which we 

 call thirst, should arise. 



Contrast this condition of the mouth with the condition of the respiratory 

 tract, in whicli the lining membrane consists of columnar epithelium and is 

 richly provided, particularly in the nose, with multitudes of mucous glands. 

 Through thi- tract air moves to and fro constantly with no sign of inducing 

 desiccation except in extreme and prolonged deprivation of water. But 

 there is one portion of this normal pathway for the air which, in the 

 absence of sufficient moisture, is peculiarly liable to become dried. It is 

 the pharynx, where the respirator}- tract crosses the digestive tract — 

 i.e., where the inbreathed air, which may be insufficiently moistened in 

 the nose, passes over surfaces of the ancient watercour-^e. Here, even with 

 nasal respiration, unpleasant feelings may be excited, if the water-content of 

 the body is reduced, and, in cases of marked thirst, the dryness of this 

 region may stimulate tii'eless swallowing motions. 



The central questions now appear: Why do not the mouth and pharynx 

 feel dry and uncomfortable under normal conditions ? and why do they feel 

 so when the body stands in need of water ? Again, a comparison of 

 conditions in the water inhabitants, in which the buccal and pharyngeal 

 regions ai'e kept moistened by the surrounding medium, with conditions in 

 the air inhabitants, in which these regions tend to be dried by the 

 surrounding medium, will offer pertinent suggestions. A characteristic 

 difference between these two animal groups is the possession, by the air 

 inhabitants, of special buccal glands. They are not present in fishes, but 

 are found in the rest of the vertebrate series from the amphibia onwards. 

 At first little differentiated, they develop in mammals into the three pairs of 

 saUvary glands — the parotid, sub-maxillary, and sub-lingual. For the 

 purpose of considering thirst in man, we may deal solely with this salivary 

 group. The action of these organs is to secrete a fluid which is normally 

 more than 97 per cent., and may be more than 99 per cent., water.* The 

 * Becker and Ludwig, ' Ztschr. f. Rat. Med.,' vol. 1, p. 278 (\^b\). 



