Cold abscesses have not only a significance of their own, but for the most part an identity. Their distinguishing feature is a _limiting membrane_, which forms whenever sufficient time has elapsed. Much has been written about it, and much error has been perpetuated with regard to it. This is the membrane formerly considered and called _pyogenic_, under the misapprehension that by it the pus or contents of the abscess were produced. I desire to emphasize in every possible way that this is a mistake. This membrane does not act to produce pus, but is rather the result of condensation of cells around the margin of the tuberculous lesion, forming, as it were, a sanitary cordon, for the absolute and definite purpose of protection against further ravages. I would suggest that the term pyogenic membrane be abolished, there being no such membrane under any circumstances, and that, this be known as that which in effect it is--namely, a _pyophylactic membrane_. _It is a protection against pus_, and were it not for its presence there would be no limit to the spread of tuberculous invasion. A lesion thus surrounded is shut off from most possibilities of harm, rarely encroaches, except by the most gradual processes, and, on the contrary, often contracts and reduces its dimensions, the watery portion of its contents being gradually absorbed and the more solid and cellular portions becoming condensed into matter which undergoes caseous degeneration, so that eventually recovery may ensue as the consequence of a metamorphosis of an original cold abscess into a caseous nodule surrounded by the old pyophylactic membrane, which is now serving as a capsule.