228 PRINCIPLES OF VETERINARY SURGERY 



begin an operation too soon, even after the patient has 

 lost all consciousness, there will be a marked resistance to 

 the pain inflicted by the knife. The cerebrum, — the most 

 sensitive organism, — is therefore first to yield to the in- 

 fluence of the molecules of inhaled vapor that are carried 

 to the cranial cavity by the blood. If the administration 

 is discontinued at this moment a few breaths of air im- 

 mediately terminates the phfenomenon and the subject 

 promptly revives, none the worse from the ordeal; but if 

 the inhalation of the vapor is continued, then the less sen- 

 sitive cells of the body begin to fall before the influence, 

 one after another. The structure next in the scale of sen- 

 sitiveness is the excito-motor apparatus, which being at- 

 tacked by the molecules of the anaesthetic cause first a 

 partial and then a total suspension of all voluntary motion. 

 When the latter occurs the vital functions are already 

 yielding, and unless the supply is curtailed at this moment 

 they too suspend activity, and death supervenes from toxic 

 apnoea, from overdosing. 



The chain of events produced by anaesthetics from the 

 very beginning to the end (death) are usually divided into 

 three stages known as the stages of anaesthesia. These are : 



I. The Stage of Excitement. — So far as the domestic 

 animals are concerned this period of anaesthesia is nothing 

 more than the natural resentment of the subject to the ir- 

 ritation of the anaesthetic vapor to the nasal mucosa and 

 skin, and a sense of suffocation produced by obstructing 

 the free admission of air into the air passages. It is but a 

 fight against the anaesthetist, the apparatus and the anaes- 

 thetic. During this period the animal struggles vio- 

 lently, — with all its might, — to escape the impending 

 discomfort. The horse will struggle against the restrain- 

 ing apparatus with the legs, strain every muscle of the body 

 and will attempt to release the head from its confinement. 



