6 XECEOSIS OF TISSUES. 



tionately narrow and unyielding opening, as, e.g. a knuckle of bowel in 

 the neck of a hernial sac. In this case, the pelding walls of the veins 

 undergo compression before the arteries, and so the return of blood may 

 have ceased long before its afflux is at all interfered with. We may 

 accordingly expect to find the mortified part greatly congested. 



All the above causes of necrosis agree in making an arrest of the 

 normal flow of blood through a parb the proximate antecedent of the 

 arrest of its nutrition and its life. But nutrition may also be disturbed, 

 apart from any interference with the circulation, in the islets of paren- 

 chyma which are included in the meshes of the capillary network. 

 Xearly all such troubles, however, are developed gradually ; the tissue 

 changes to which they give rise contrast with the necrotic processes in 

 the slowness with which life is extinguished. They will form the sub- 

 ject of succeeding chapters. The only true necroses which come under 

 this head are those involving organs, which, like the cartilages and the 

 cornea, are absolutely non-vasoular, when these are wholly sejDarated 

 from their connexion with neighbouring parts by suppuration. The 

 circulation through those vessels which formerly supjDlied the organs 

 with nutrient matter is not arrested ; it is only that transit of pabu- 

 lum from cell to cell, on which both cartilage and cornea depend for their 

 nutrition, which is brought to a stop. 



'Eo allusion has yet been made to those cases of necrosis in whicli 

 the death of a part is caused by chemical or mechanical agents operating 

 from without, as, e.g. crushing, concussion, desiccation, corrosion, and 

 septic poisons. In these cases, we have to do with violent disturbances 

 of molecular arrangement which are incompatible with the continuance 

 of vital activity in any form. 



§ 10. A striking pecnliarity of living tissues is their ])Ower 

 of retaining their form and characters in fluids capable of dis- 

 solvino; albuminous substances and their derivatives. Hence it 

 is a sure sign of death \vhen the tissues are no longer able tc) 

 -withstand the solvent action of such fluids. Tliis feature is 

 common to all o'^noTcnous chano;es, and it is one which is early 

 recognisable ; the dead part loses its normal elasticity, its tiur/or 

 rifalis ; it becomes flabby, soft, and doughy. Now, if a too 

 rapid evaporation from the surface of the mortified pai't is pre- 

 vented (and this office is performed by the cuticle, when present), 

 we find that the 81 per cent, of water contained in the normal 

 organism, together with the water which is set free on the spot 

 by the very act of decomposition, is amply sufficient to dissohe 

 all the solids of the body with the exception of the bones. AVo 

 already know, moreover, that in most cases of gangrene the 

 vessels of the mortified part are unusually gorged with blood. 



