CH. III.] EXTERNAL SENSATIONAL ACTIONS. Ill 



takes place, for the most part, through nerve- actions (183), for 

 the external sensation of each impression is seldom perceived, 

 only the vivid external impressions being felt, when the 

 painful actions of external sensations caused in it are contra- 

 natural. Thus arises the violent peristaltic action of the 

 stomach and bowels from griping purgatives, poisons, inflamma- 

 tions; hence also the spasm of the stomach from similar causes, 

 or from flatulency, or indigestible food painfully pressing the 

 stomach. The muscles of the limbs participate largely in such 

 contra-natural movements of the stomach and bowels, as is 

 shown by the paralysis which supervenes on violent colic. 

 This takes place in virtue of the sympathy between the sentient 

 actions of muscles, but the question belongs properly to the 

 pathology of organisms. 



213. The liver and gall-bladder are little susceptible of ex- 

 ternal sensations : an external impression is only perceived when 

 it is remarkably vivid, and also contra-natural, or painful (198). 

 Violent pain in the region of the liver exercises an injurious 

 influence on the secretion and excretion of bile (175). Other 

 conceptions, like external sensations, have a manifest influence 

 on most of the proper viscera of the body by sentient actions 

 on their nerves. 



214. Ordinary external impressions on the lungs are rarely 

 felt; when, however, they experience tickling or pain, the 

 respiration is partly accelerated, partly rendered contra-natural 

 and spasmodic, as in cough ; in cases of this kind the sentient 

 actions are from sympathy (98, 165). Respiration is not excited 

 solely by irritation of the nerves of the lungs, but also by 

 irritation of the diaphragm, and of the respiratory muscles 

 of the thorax. 



215. The kidneys also are susceptible only of extraordinary 

 and unnatural external impressions ; spasm is almost the only 

 example of sentient action from external sensations that can be 

 deduced. It is otherwise with the bladder. 



216. The organs of the senses, considered as mechanical 

 machines, are moved by external sensations in various ways, 

 having reference to their particular functions. When a sound 

 enters the ear, and is heard, the muscles which stretch the 

 tympanum are so put into action, as to render the latter tense 

 in accordance with the tone : so when light enters the eye, and 



