496 MATER i A MEDIC A AND THERAPEUTICS. 



centre as to cause the gasping movements of breathing familiar 

 under the circumstances. In other words, local stimulants may 

 become powerful general stimulants. 



b. Local Anaesthetics. Pursuing an exactly opposite line of 

 action, we can readily diminish the sensibility of the origins of 

 nerves until their power of receiving impressions is lost, and 

 thus remove sensations by preventing the very contact of the 

 influence with the nervous system. The measures which have 

 this effect are called local anaesthetics (av, without, and afae-riffis, 

 sensibility), or if pain be relieved, local anodynes (av, 

 without, and oSi^vrj, pain}. Some of these agents directly depress 

 the nerve fibrils, such as Belladonna, Aconite, Veratria, and 

 Opium ; and Ether, Alcohol, Chloroform, Carbolic Acid, Volatile 

 Oils, and Cantharides, when their application is prolonged. 

 Moderate cold, especially such as is induced by evaporation, is 

 decidedly anaesthetic; and Ether, Spirits, Acetic Acid, Water, and 

 various Saline solutions, e.g. of Chloride of Ammonium, possess 

 this property. Prolonged or extreme cold directly reduces the 

 functions of the nerves, causing first numbness, and then absolute 

 anaesthesia. Warmth reduces, and extreme heat destroys, 

 the irritability of the nerves. Other anodynes act partly 

 or wholly through the vessels. Thus moderate heat relieves 

 pain partly by dilating and relieving the blood vessels, and by 

 increasing the blood-supply, the osmosis, and the migration of 

 corpuscles in the tissues an effect which is assisted by moisture, 

 as familiarly seen in poultices. Cold partly acts by reducing 

 excessive blood supply. The galvanic form of electricity 

 often removes pain very quickly, probably by acting on the 

 nerves, muscles, vessels, and even the metabolism of the 

 part. 



The influence of local anaesthetics and anodynes is not con- 

 fined to the sensorium. With the arrest of sensation, the 

 whole brain passes into a state of rest, and sleep readily occurs. 

 The in-travelling impressions being reduced in strength, the 

 spinal and medullary centres through which they pass, or into 

 which they previously radiated, are no longer excited, and the 

 action of the organs, such as the lungs and heart, becomes more 

 automatic, and, as a rule, but not invariably, more quiet. 

 Thus, as with local irritants, we possess in local anaesthetics 

 and anodynes, a powerful means of influencing the functions of 

 the highest centres, the visceral centres, and the viscera them- 

 selves. In other words, local sedatives may become powerful 

 general sedatives. 



c. All these measures act upon the peripheral structures. The 

 trunks of the afferent nerves may also be affected so as to inter- 

 fere with the convection of the impressions. Opium, and 



