No sharp line can be drawn between these diseases and some of the preceding, inasmuch as it often depends on the external conditions whether necrosis is a dry-rot, in the sense I employ the term here, or a wet-rot, when it would come under the preceding category.
The usual necrosis of the injured cortex occursdrying up, shrivelling, and consequent stretching and cracking of the dead cortex on the wood beneath.
Such frost-cracks, sun-cracks, &c., may then be slowly healed over by callus, but if the conditions for necrosis recur the crack may be again opened, or if Fungi, &c., interfere with occlusion, the healing is prevented; in such cases the local necrosis may give rise to cankers.
General attacks of leaf-diseases invariably lead to starvation and necrosis of twigs, and similarly with the ravages of caterpillars and other insects.
Drought and consequent defoliation result in the same, and these considerations help us to understand how old-established trees in parks, &c., apparently in good general health, become stag-headed by the necrosis of their upper twigs and smaller branches: the roots have here penetrated into subsoil or other unsuitable medium, or some drainage scheme has deprived them of water, &c., and a dry summer just turns the scale.
The caseous necrosis of the implicated mass of lung tissue, and indeed of tubercles generally, is held to be, in great measure, the result of the necrotic influence of the secretions from the bacillus.
Thus workers in lead suffer from the effects of this substance as a poison, those who work in phosphorus are liable to necrosis of bone and fatty degeneration of the blood vessels and organs, and the many occupations in which dust is inhaled (coalmining, stone-dressing, steel-polishing, &c.; fig.
This condition is not so frequently seen in the more highly differentiated cells, but may follow necrosis of secreting cells, as is found in the kidney, in corrosive sublimate poisoning and in chronic nephritis.
Owing to liability to necrosis, the permanent retention of such a mass of dead bone would be dangerous; and the antlers are consequently shed annually (or every few years), to be renewed the following year, when, till the animal becomes past its prime, they are larger than their predecessors.
Thus, to mention examples, diphtheria toxin produces inflammatory oedema which may be followed by necrosis; dead tubercle bacilli give rise to a tubercle-like nodule, &c. Furthermore, a bacillus may give rise to more than one toxic body, either as stages in one process of change or as distinct products.
For the chronic form of industrial poisoning in the manufacture of lucifer matches - a form of necrosis, known in England as " phossy jaw " and in France as " mal chimique," a localized inflammatory infection of the periosteum, ending with the death and exfoliation of part of the bone - see Match.