LOCAL STIMULANTS. 495 



the tissues folio-wing exhaustion. Sleep bears a definite rela- 

 tion to work, food, and the time of the day, and brings rest and 

 refreshment to the exhausted system. 



II. PHARMACODYNAMICS. 



When we come to consider how far the nervous system is 

 under our influence, we enter upon a field of enormous propor- 

 tions, of which we can take but a few examples. 



1. Sensation. "We have a remarkable power over both common 

 sensibility and the special senses, increasing or diminishing 

 their activity at our pleasure, by means respectively of local 

 stimulants and local anesthetics. 



a. Local stimulants. This name is given to a great and 

 mixed group of agents, which increase common sensibility or 

 common sensation so much as to cause pain. The majority of 

 them act directly upon the nerve jibrils in the tissues, such as 

 extreme heat, extreme cold (for a time), faradic electricity, and 

 many drugs, including : Iodine and Bromine ; Alcohol, Ether, 

 and Chloroform, when the vapour is confined ; Carbolic Acid and 

 Creasote"; volatile oils, e.g. Turpentine, Cajuput, Menthol, 

 Thymol ; acrid essential oils, e.g. Mustard and Mezereon, and 

 Cantharides in the first stage. Mineral Acids and Ammonia; 

 Metallic salts, such as those of Silver, Lead, Zinc, Antimony, 

 Mercury, Arsenic, and Copper, also stimulate the nerves and 

 cause severe pain, but not when supplied in sufficient strength 

 to interfere markedly with the vessels and protoplasm of the 

 part as caustics or astringents. Possibly some local stimulants 

 act primarily upon the vessels, and many of them no doubt 

 excite the circulation as well as the nerves. It must 

 be carefully noted that the effect of local irritation on the 

 sensory apparatus is really a central one. The sensation of 

 pain, although it may be referred to the periphery, is a cerebral 

 state. It therefore affords us a means of rousing the highest 

 centres. What is even more important therapeutically, the 

 whole of the impression conveyed from the irritated spot does 

 not become converted into a painful sensation or act of con- 

 sciousness. A portion of it, whilst traversing the grey matter 

 of the spinal and medullary centres en route, disturbs these 

 and causes reflex impulses, which rouse the muscles and viscera. 

 In this way sensory, and especially painful impressions are 

 powerful and readily available means of stimulating not 

 only consciousness but the cardiac, vaso-motor, and respira- 

 tory centres, and through them the great viscera themselves. 

 Thus the cold douche produces a sensation of cold referred to 

 the part, rouses consciousness and so excites the respiratory 



