MULTICELLULAR ANIMALS 237 



irritability from which recovery is prompt and without injury or 

 poisonous effect. No perfect anaesthetic has yet been devised, but 

 progress is being made; modern Chemistry has lessened the in- 

 jurious action of anaesthetics without impairing anaesthetic qualities. 



The discovery of the perfect anaesthetic awaits the complete solu- 

 tion of the problems of the physico-chemical nature of excitation 

 and transmission. Present views are that the action of the anaes- 

 thetics now employed is to stabilize the surface membrane of the 

 nerve fibre so that the electrical and metabolic phenomena that 

 constitute excitation and transmission are prevented. 



If we employ as an anesthetic some given substance, for instance 

 ether, on a series of animals ranging from Protozoa to mammals, 

 it is found that in general the animals lowest in the scale are least 

 readily anaesthetized. Thus not only are the nervous mechanisms of 

 the higher animals structurally more complicated and efficient but 

 also their properties of excitation and transmission are more highly 

 refined, more sensitive to environmental conditions and more read- 

 ily extinguished by ansesthetics. 



A clinical terminology of anesthesia that is generally employed 

 has the virtue of commonly accepted usage and accuracy in a broad 

 sense, but may be misleading unless the facts are understood. Thus 

 in the case of a major operation the patient is given what is called a 

 general anaesthetic. But the actual efFect of such an anaesthetic is 

 merely the anaesthesia of certain local regions of the brain. Should 

 this local anesthesia extend to complete and general anesthesia of 

 the brain, as sometimes accidentally happens, the centres that con- 

 trol respiration and heart beat are inactivated. So a clinical general 

 anesthesia is in reality a local anesthesia of the regions of con- 

 sciousness in the brain. On the other hand, when in minor opera- 

 tions a so-called local anesthetic is used, the effect is a general 

 anesthesia of all tissues within the region of its diffusion. Thus it is 

 local only in the sense that its effect is limited as to region and not 

 as to tissues affected. 



