_Indication for Local and Spinal Anæsthesia._--The foregoing has been written in reference to inhalational anæsthesia solely. Mr Wood has indicated in chapters XX. and XXI. the class of case in which the methods he describes are to be preferred. CHAPTER XX. LOCAL ANÆSTHESIA. By the term local anæsthesia, or more correctly _local analgesia_, is meant the loss of sensibility to painful stimuli without loss of general consciousness. It may be induced in a considerable number of ways, but for practical purposes there are only four methods of value:--(1) by infiltration of the tissues to be operated upon by a solution of the drug, (2) by injecting the solution into or around the nerve trunks supplying the part, (3) by painting the solution on a mucous surface, and (4) by the application of intense cold. The last method has only a limited application. The method of injecting the anæsthetic into the blood vessels of the part is still in the experimental stage and is not to be recommended for general use. It is advisable first to consider the behaviour of the principal drugs which are employed. Cocaine. This was the first drug to be widely used for the production of local anæsthesia. It is an alkaloid occurring in the leaves of _Erythroxylon Coca_. It is only slightly soluble in water--about 1 in 1300, but the hydrochloride of cocaine is freely soluble, and it is this salt that is commonly used for aqueous solutions. The solutions do not keep well, and should be made up shortly before being used. The drug is decomposed by boiling. ACTION.--When a solution of cocaine is injected into the tissues, the sensory nerve endings become anæsthetic over the area into which the drug penetrates, direct paralysis of the nerve terminals being produced. When it is injected into or around a nerve trunk it blocks the transmission of nerve impulses. When it is applied locally to a mucous membrane, it produces, besides a loss of sensation, a feeling of constriction and a distinct pallor and contraction of the vessels, which point to a local action on the vessel walls. The drug is very frequently applied to the eye. There it produces not only local anæsthesia, but also contraction of the conjunctival vessels, and this is followed by dilatation of the pupil and often by partial loss of the power of accommodation.