46 OPERATIVE TECHNIQUE. 



tile anaesthetics must penetrate by the respiratory mucous membrane. 

 Injected into the tissues or veins, fixed anaesthetics traverse the 

 pulmonary capillaries without sensible change, and act promptly on 

 the nerve-centres ; volatile anaesthetics, similarly administered, escape 

 in large measure through the walls of the pulmonary vessels, are 

 expired, and fail to reach their destination in sufficient quantity to pro- 

 duce much effect. On the other hand, volatile substances, introduced 

 in a state of vapour into the respiratory tract, are freely absorbed by 

 the blood circulating in the lungs, which blood, passing thence to the 

 left heart and general arterial syitem, rapidly produces anaesthesia. 



Anaiisthetics administered by the respiratory tract produce a series 

 of phenomena in the following order :■ — (i) 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 rpembrane 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 sensa- 

 tion, excitement, 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 exten- 

 sive, 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 in 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 fiank 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 

 intjoxication occurs. Vision is no longer co-ordinated, while the pupils 

 remain contracted and immobile. x\s sensation is lost in the various 

 upl^ions 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 



