Foucault discusses Mettray and the carceral archipelago or network that formed around it. It is important that not all inmates were criminals, and that this is a key feature.
1. The carceral continuum: so many gradations of criminality, and so much discipline (which is distinct from punishment).
2. The carceral network produces delinquents, trained in their discipline.
…[T]he delinquent is not outside the law; he is, from the very outset, in the law, at the very heart of the law, or at least in the midst of those mechanisms that transfer the individual imperceptibly from discipline to the law, from deviation to offense. (301)
But it is not on the fringes of society and through successive exiles that criminality is born, but by means of ever more closely placed insertions, under ever more insistent surveillance, by an accumulation of disciplinary coercion. In short, the carceral archipelago assures, in the depths of the social body, the formation of delinquency on the basis of subtle illegalities, the overlapping of the latter by the former and the establishment of a specified criminality. (301)
3. It naturalizes the legal right to punish, equates punishment and justice.
4. It creates a new form of law, one that decides what is or is not deviant.
5. It depends upon surveillance, observation, and thus knowledge-power. (Notice here the connection to the disciplines of the human sciences.) There is power and judging everywhere, in many institutions, all the time.
6. This connection to knowledge and discipline is what gives the prison its solidity. See on 307 the list by the anonymous writer, of all the institutions that support the [prison industrial complex] and how they are integrated to all of society. The “carceral city” is all of these mechanisms and institutions, spread like a network and effectively structuring society.
The whole book is available in PDF at Monoskop.