40 GENERAL ACTIONS OF DRUGS 



(b) General Ancesihetics. 



Ether 



Chloroform 

 Nitrous oxide 

 Ethylene dichloride 

 Methylene dichloride 

 Schleich's mixture 



Of slight value in Veterinary medicine 



AnsesthetiGS are agents which abolish sensation generally 

 or locally. It is thought that the general anaesthetics act 

 directly on the nerve cells. Ansesthetics— like narcotics 

 generally — first stimulate and then depress the nerve centres, 

 but depression is by far their most salient and useful effect. 



Anaesthetics 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. Anaesthesia is commonly described in three 

 stages. (1) The first or stimulant stage is exhibited by excite- 

 ment and struggling, owing in part to fright and in part to 

 stimulation, first of the higher cerebral centres, and then of 

 the lower cortical centres. 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 circulation aud respiration are temporarily stimu- 

 lated. Stimulation now ceases and depression of the 

 cerebrum, together with the motor, sensory and reflex spinal 

 centres, appears, and ushers iu the (2) ancesthetic stage, char- 

 acterized by muscular relaxation and complete abolition of 

 consciousness, sensation and motion. Between these two 

 stages — the stimulant and anaesthetic — there sometimes 

 occurs a transient state in which sensation is lost before 

 consciousness. This has been styled the anodyne stage. 



Finally the (3) paralytic stage ensues, accompanied by 

 depression and then paralysis of the three great vital 

 medullary centres controlling the circulation and respiration, 

 together with that of the lowest reflex centres, so that invol- 

 untary micturition and defaecation occurs. The animal dies 

 of a combination of vasomotor, heart and respiratory failure. 



