DRUGS ACTING ON THE NERVOUS SYSTEM 35 



gesics. Pain is due to irritation of any sensory nerve, or the sensory tract 

 in, the spinal cord, or of the sensory centres in the brain. Pain may be 

 relieved by paralyzing any portion of this path and destroying connection 

 vrith the perceptive centres in the brain. 



(1) Anodynes, by reason of their action on the brain. 

 Codeine Alcohol Cannabis Indica 

 Morphine Anesthetics Gelsemium 

 Opium Chloral Bromides 



(2) Narcotics. — This term is a broad and somewhat inclusive one. 

 Narcotics embrace drugs which depress the brain and cause sleep (hyp- 

 notics or soporifics) and stupor (some anodynes and anesthetics), and 

 finally paralyze the respiratory and circulatory functions. The follow- 

 ing may be included in this group : 



Opium Anesthetics Cannabis Indica Stramonium 



Alcohol Chloral Belladonna Hyoscyamus 



(a) Hypnotics or Soporifics (drugs causing sleep) : 

 Opium Bromides Paraldehyde Sulphonal 

 Morphine Cannabis Indica Urethrane Trional 

 Chloral (Of little importance in veterinary 



practice) 



Uses. — Hypnotics are not of much value in veterinary medicine by 

 simply promoting sleep. Their general sedative and anodyne actions are 

 utilized in relieving motor excitement (spasms) or sensory excitement 

 (pain) . 



(b) General Anesthetics. 



Ether Nitrous oxide Methylene bichloride 



Chloroform Ethylene dichloride 



(Of slight value in veterinary medicine) 



Anesthetics are agents which abolish sensation generally or locally. 

 It is thought that the general anesthetics act directly on the nerve cells. 

 Anesthetics — -like narcotics generally — first stimulate and then depress 

 the nerve centres, but depression is by far their most salient and useful 

 effect. 



Anesthetics destroy the functions of nerve centres in the cerebrum 

 and spinal cord, and so abolish pain, sensation and reflex action. The 

 law of dissolution is exemplified in their action. Anesthesia is commonly 

 described in three stages. (1) The first or stimulant stage is exhibited 

 by excitement and struggling, owing in part to fright and in part to irri- 

 tation of the respiratory tract by concentrated vapor. There are also 

 coughing and choking in this Stage, following the local irritation of the 

 vapor on the respiratory tract. There may be vomiting, and the circula- 

 tion and respiration are reflexly stimulated. Stimulation now ceases, 

 and depression of the cerebrum, together with the motor, sensory and 

 reflex spinal centre,?, appears, and ushers in the (2) anesthetic stage, 

 characterized by muscular relaxation and complete abolition of conscious- 



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