( H. II.] EXTERNAL DIRECT NERVE-ACTIONS. 247 



the skin causes redness, it is the sentient action of an ex- 

 ternal sensation at the irritated part; but when the same takes 

 place after decapitation, it is the direct nerve-action of an 

 external impression on the terminations of the capillaries of 

 the skin (462) . When an irritation of the mucous membrane 

 of the nares, causes a flow of mucus, a sentient action of an 

 external sensation takes place; but if the same occurs after 

 removal or destruction of the brain, it is a direct nerve-action 

 in the capillaries, or the glandular tissues of the mucous 

 membrane. 



On account of the deficiency of nerves in the insensible 

 fibro-serous tissues of the thorax and abdomen (208), no other 

 nerve-actions occur than those produced in the sensitive nerves. 

 Thus a repelled transpiration from cold excites the capillaries 

 of the pleura to contract, and thereby induces congestion and 

 inflammation, although no sensation is excited by this external 

 impression ; for the pain results from the inflammation (208, 

 note) . 



465. Although the broad muscles and muscular tissues are 

 sensitive (171), and an external impression excites their natural 

 functions as sentient actions (as, for example, when an irritation 

 causes spasmodic action of the diaphragm) ; yet it often happens 

 that the excited functions are the direct or indirect nerve- 

 actions of the same external impression, or even mechanical 

 results of the direct nerve-action (447, 464). Thus, the 

 external irritation which excites the diaphragm to motion, and 

 that which irritates the muscular tissue of the glands, when 

 they pour out their secretions, are very seldom felt. It is the 

 same with the indirect nerve-actions in the membranes devoid 

 of muscular tissue ; although they sometimes arise from ex- 

 ternal sensations, yet general observation shows, that small 

 suff'usions, inflammations, congestions, induration, effusion, &c., 

 take place in the pleura, peritoneum, &c., which necessarily 

 result from external impressions on those tissues that are 

 altogether unfelt; just as occurs in various diseases, namely, 

 erysipelas, catarrh, cough, pleurodynia, cutaneous eruptions, &c. 



466. The large muscular viscera, particularly the oesophagus, 

 stomach, and intestines, are not only directly excited by ex- 

 ternal impressions, but in virtue of their peculiar structure, and 

 by means of their direct nerve-actions, are so excited through- 



