SOME HARDENING ACCENTS. 373 



the body, but personally I think the developing pulp in teeth 

 should be included, for if anything it is more easily changed than 

 the brain. 



It is well known that alcohol cannot — or, rather, should not — 

 be used for central nervous-system hardening, and least of all 

 should it be used for the white substance, as it fails to harden 

 properly. Moreover, a large share of the fatty constituents of the 

 medullary substance is extracted by the alcohol, to be precipi- 

 tated later in a crystalline form, and so injure the tissues. Thin 

 tissues and membranes are also best hardened by some other fluid 

 on account of their curling up. Intestines and tracheae are good 

 examples of this. 



Let us briefly consider the question of using absolute alcohol, 

 as ordered by some. First, remember we must only use the very 

 thinnest and smallest pieces of tissue, and therefore must guard 

 against curhng up. Second, we know the moment the alcohol 

 touches the specimen its action is to coagulate ; in fact, so 

 quickly does it act, that only the external layer of cells is 

 affected, and the deeper ones are not hardened at all. Why is 

 this ? Because we have placed a coagulated mass first, and this 

 acts as a barrier, if I might so term it, and prevents the agent 

 penetrating. This has been proved in the case of bichloride of 

 mercury and its action as a germicide, by its forming a capsule 

 around the bacilli which the agent cannot penetrate, and if the 

 germs are allowed to recover themselves, a chemical action is pro- 

 bably set up which will dissolve the coagulated capsule and set at 

 Hberty the bacilli to resume their work. 



We know the action of carbolic acid, so let us take it as an 

 example from dental surgery; but unfortunately it is not con- 

 sidered as it should be, for if we use the pure acid we produce a 

 caustic action, an eschar results, and the drug scarcely penetrates 

 except to the first layer of tissue. Try it on the skin. We get no 

 action unless we remove the eschar and again apply, and so on. 

 But let us use a solution of a low strength, and we can get it to 

 penetrate to such a degree as to injure the deeper layers. In a 

 tooth it will penetrate in many cases, irritate the pulp, and often 

 cause its death. 



From these facts it will be sufficient to show the restriction 



