DIGESTION 107 



bloodvessels of the scalp. But the vaso-motor condition is 

 sympathetic with the disturbance of the brain ; and the special 

 urgency or efficiency of the messages from the skin results 

 from their being delivered into excited brain-tissue. Nausea 

 and headache are equally symptoms of the irritability of the 

 brain caused by the motion of the ship. In one case messages 

 from the stomach, in the other case messages from the scalp, 

 acquire undue importance, owing to the agitated condition of 

 the brain-tissue through which they pass. Not uncommonly 

 the voyager, who wakes in the morning reconciled to the 

 changes of pressure which he has experienced while recumbent, 

 finds, when he stands upright, that the base of his brain is 

 as sensitive as ever. Visual sensations also contribute to the 

 brain-disturbance. So, too, do the movements of endolymph in 

 the semicircular canals (cf. p. 335). It is, indeed, possible that 

 this last factor is more important than the variations in 

 pressure on the surface of the brain. Probably it accounts 

 for the after-image of rolling which almost everyone experiences 

 for at least a day after leaving the ship. Its cause being 

 cerebral, the tendency to sea-sickness can be controlled by 

 drugs which, like the bromides, chloral, alcohol, etc., deaden 

 the brain. 



Salivary Glands. The secretion which accumulates in 

 the mouth is the combined product of the sublingual, sub- 

 maxillary, and parotid glands. It is a very thin, watery 

 solution containing not more than 0-5 per cent, of solid sub- 

 stance. If red litmus-paper is moistened with saliva, it becomes 

 blue, showing that the secretion is alkaline. It contains a 

 ferment, ptyalin, which digests starch. The action of this 

 ferment can be demonstrated by holding in the mouth for half 

 a minute some warm starch mucilage boiled arrowroot, for 

 example. It quickly loses its viscidity owing to the conversion 

 of starch into sugar. Chemically this change may be demon- 

 strated by adding iodine-water to a specimen of the starch 

 before and after action. Before the starch is taken into the 

 mouth the iodine turns it blue (a characteristic reaction for 

 starch). After it has been exposed to the digestive action of 

 the saliva, iodine fails to colour the mixture, which now contains 

 no starch. All the starch has been converted into dextrin 

 and sugar. If unboiled arrowroot is placed in the mouth, 



