160 DEPRESSANTS TO CEN'i'RAL NERVOUS SYSTEM 



Properties of General Anesthetics. In order to be of use as 



a general anesthetic, a drug must be very rapidly absorbable, pro- 

 duce anesthesia very quickly, and be rapidly eliminated. Further- 

 more, it should produce muscular relaxation as well as complete un- 

 consciousness, i.e., abolish cerebral and spinal activity without dan- 

 gerous depression of the vital centers in the medulla or permanent 

 derangement of the central nervous system. Since these drugs are 

 very volatile and their vapors are absorbed rapidly by the lungs, their 

 administration by inhalation is preferred, as it is more easily 

 controlled, but, on the other hand, a sufficient amount given per os, 

 rectum or intravenously will produce the same action. 



The principal members of the group of anesthetics are chloro- 

 form, ether, nitrous oxide, ethyl chloride and ethyl bromide. 



Anesthesia. For the convenience of study the action of anes- 

 thetics is usually divided into three stages, but since these stages are 

 only different degrees of the same action and there is no real line of 

 demarcation between them, it is quite optional where the line of 

 division is made. In himian medicine four stages are usually 

 described: stimulant, narcotic, anesthetic and paralytic. Since the 

 first two cannot be easily differentiated in the domesticated animals, 

 it seems well to discuss anesthesia in the three stages, stimulant, 

 anesthetic and paralytic. 



Stimulant. This is characterized by stimulation of the cere- 

 brum and cerebellum and is manifested by great excitement and 

 involuntary movements. The first few inhalations may produce a 

 sensation of asphyxia, so that the respirations are often stopped 

 voluntarily. The pulse and respirations are soon accelerated and the 

 pupils dilated. During this period the horse often whinnies and the 

 dog barks. All the above symptoms are due to excitement or stimu- 

 lation. There is usually an increased secretion of saliva, mucous 

 and tears, probably due to the irritant action of the drugs upon the 

 parts involved. 



Anesthetic. This is the condition it is desired to produce and 

 maintain. It is characterized by complete paralysis of the brain, 

 and motor reflexes in the spinal cord and depression of all the cen- 

 ters in the medulla. Consciousness, sensation and most of the re- 

 flexes are lost, the corneal reflex being one of the last to go. All 

 voluntary muscle is relaxed but smooth muscle is not so apt to be 

 affected, although there may be some relaxation of the sphincters. 

 The pulse is slow, full and soft on account of lowered blood pressure. 

 The respirations are regnlar but shallow. Temperature falls on 

 account of lessened heat production and increased heat loss. Since 

 the patient will not feel pain and the reflex activity is so lowered 

 that the heart will not be reflexly inhibited by the shock of the 

 operation, this is the period during which it is desired to operate 

 safely. 



