THIRST. 349 



may be produced, even after the eyes are closed. This state, which 

 is known as secondary vertigo, may be produced by steadfastly looking 

 at quickly moving objects, by after-impressions caused by moving ob- 

 jects, by looking down from great heights, and by rapid unusual move- 

 ments of the body. Rotatory movements will produce vertigo, even 

 though the eyes be closed. 



Lastly, to the muscular sense, combined with certain feelings of 

 pressure and strain about the joints, seated probably in the ends of 

 the bones which support the cartilages, and in the ligaments which 

 tie the bones together, we are indebted for our notions of resistance 

 and of weight; these, however, are not the results of a simple sen- 

 sation, but of inferences drawn from the perception of the effects of 

 gravity and force. The muscular sense appears to be keenly exalted 

 in somnambulists, as well as the power of muscular combination and con- 

 trol. It has been suggested, that the special sensorial centre of this 

 muscular sense is in the cerebellum (page 290). This, however, seems 

 improbable, for when it is lost in any particular muscle or muscles, 

 the common sensibility disappears with it ; hence, it has been regarded 

 as only a modification of this common sensibility. 



The internal sensations dependent on states of the organs of vege- 

 tative life, are exceedingly varied, in accordance with the number and 

 variety of those organs themselves. Many of them must be produced 

 by impressions made on the ultimate ramifications of the sympathetic 

 nerve, or of branches of the cerebro-spinal nerves which are asso- 

 ciated with these in their distribution. They are chiefly referable 

 either to the digestive organs, the organs of circulation, or the respi- 

 ratory organs. 



The internal sensations connected with the digestive system, are 

 thirst, hunger, satiety, and nausea. Thirst is principally a local sen- 

 sation, being chiefly referred to part of the tongue and palate ; but it 

 is evidently dependent upon a general state, for it may be distin- 

 guished from mere dryness of the mouth and fauces. This latter 

 condition, which is produced by sleeping with the mouth open, is 

 quickly relieved by shutting it, or, at most, by merely moistening the 

 mouth with water, and immediately emptying that cavity. True thirst 

 is not so relieved, but only by copious drinking, or by continual im- 

 mersion of the body in water, even though this be salt. The general 

 condition on which thirst immediately depends appears to be a defi- 

 ciency of water in the blood ; and as the blood is the source of all the 

 secretions, these are everywhere diminished, and those of the fauces and 

 mouth are necessarily deficient. An additional cause of dryness in 

 the throat especially, is, that its surface is momentarily exposed to 

 evaporation in the respiratory act, so that thirst is consequently local- 

 ized in the fauces. The sense of dryness of those parts is communi- 

 cated to the sensorium by impressions conveyed along the fifth, the 

 glossopharyngeal, and the pneumogastric nerves, the two latter of 

 which are probably more sensitive than other nerves to the condition 

 of dryness. Not only the want of water in the blood produces this 

 feeling, but the excess of saline matter will likewise cause it, as is no- 

 ticed after taking much common salt with the food, drinking salt 



