ANESTHESIA. 43 



in the lungs, which blood, passing thence to the left heart and general 

 arterial system, rapidly produces anaesthesia. 



Anaesthetics administered by the respiratory tract produce a 

 series of phenomena in the following order : — -(1) period of excitation ; 

 (2) period of anaesthesia or surgical period ; (3) period of collapse 

 or intoxication. 



The period of excitation, due firstly to the action of anaesthetic 

 vapours on the nerve terminations in the mucous membrane of the 

 upper respiratory tract and paralysis of the cerebral inhibitory centres, 

 and afterwards to the action of these vapours on the great nerve- 

 centres themselves, is characterised by perverted sensation, excite- 

 ment, and hyperaesthesia of sense organs. Violent struggling occurs, 

 respiration and circulation become accelerated, the mucous membranes 

 injected, and the pupil dilated ; the heart's action, however, soon 

 slows, respiration becomes easier, more regular, and more extensive, 

 the pupil contracts, excitement diminishes, and sleep commences. 

 During this period anaesthetics may, in animals predisposed to such 

 complications, produce respiratory or cardiac syncope or asphyxia 

 from spasm of the glottis. 



The period of anaesthesia is characterised by suspension of 

 activity in the nerve-centres, i.e. the cerebral lobes, medulla oblongata, 

 and mesocephalon. The animal is plunged into artificial sleep. The 

 excito-motor centres are paralysed, the muscles relaxed, the limbs 

 when raised fall inertly. Respiration is slow, the movements of 

 the chest wall are diminished, but those of the flank more marked 

 than ordinary. The heart is accelerated, on account of paralysis 

 of the moderator centre, but the pulse remains regular and full up 

 to the moment when intoxication occurs. Vision is no longer co- 

 ordinated, while the pupils remain contracted and immobile. As 

 sensation is lost in the various regions reflexes cease. 



Sensation does not disappear simultaneously in all tissues and all 

 regions, being last retained by organs under spinal control. The 

 limbs and trunk are first affected, then the organs of sense and those 

 supplied by branches of bulbar origin, finally those supplied by the 

 sympathetic system. The nasal, buccal, auricular, and genital mucous 

 membranes exhibit reflexes long after other parts have become 

 insensitive. Even when anaesthesia appears complete, struggling 

 often results immediately the knife touches the diseased region. 

 This is due to certain diseased tissues retaining a morbid sensibility 

 even after neighbouring healthy tissues have entirely lost sensation. 

 Inflamed parts especially retain their sensibility long after healthy 

 tissues. Once established, anaesthesia can be maintained by very 



