The poison may be drawn into the lungs with the air or swallowed in contaminated water ; but much uncertainty exists with regard to the laws which govern the transmission of the infective matter. Old cesspools and drains appear to preserve the contagium for a long time in a state of active virulence, but there is no proof that the poison can be generated spontane ously from ordinary filth. The distemper may originate in a district under one set of conditions and be distributed under other and different conditions. There is no doubt that insanitary surroundings tend to favour the spread of the disease ; still it is probable that other influences also regulate the diffusion of the infection ; for when an outbreak occurs in any district, it is not always in the poorest and least cleanly localities—in parts, that is, where the disease would be expected to be most active—that the largest number of cases occurs.
In many outbreaks certain faulty conditions, such as polluted water supply, long standing accumulation of excrementitious matters, and imper feet sewerage and drainage generally, are found to be common to all the dwellings in which the disease appears. These sanitary deficiencies are then held to furnish an explanation of the source of the infection. In other cases no such common conditions can be discovered, and the origin of the outbreak is less easy to account for. This was the case in an epidemic of diphtheria which occurred at King's Lynn, and was reported on by Dr. Airy. Here personal conveyance of the disease was positively excluded in the majority of cases. The milk was not at fault. The water-supply, the system of drainage, and the method of disposal of the excrement were in sufficient, either singly or together, to explain the distribution of the infec tion. It was, however, noticed that excavations had been in progress in the mud of the ancient river-bed and of a creek which had once been a sewer in connection with the town. Dr. Airy suggests that by this means " long buried germs of some indigenous diphtheria, causing microzymes," may have been disengaged ; and that these carried amongst the inhabitants, and aided by season and atmosphere, may have given rise to the out break.
Diphtheria is no doubt the consequence of a specific poison, however this may originate. The essence of the disease has been attributed to spherical bacteria (micrococci), which have been discovered swarming in the false membranes and exudations from the inflamed mucous surfaces ; but as similar bacteria have been found in the secretions thrown out by ordi nary non-specific stomatitis, too much importance must not be attributed to the presence of these organisms. The real nature of the virus has yet
to be discovered. The disease with which diphtheria has the closest affin ity appears to be scarlatina. Epidemics of the two disorders are frequently seen to prevail in the same neighbourhood at the same time, and it was once supposed that the exciting causes of the two diseases were the same. It is now, however, acknowledged that they have no mutually protective power ; and there is no evidence that the contagion of diphtheria has ever given rise to scarlatina.
Morbid Anatomy.—When the pharynx is examined the changes found on the inflamed mucous membrane are as follows : the surface becomes hypermmic and swollen, and after a few hours is covered with a whitish or yellowish layer which adheres closely to the mucous membrane beneath it, fitting accurately into every depression of the surface. The layer when first formed cannot be removed ; but as it increases in extent and thick ness, it gradually becomes tougher, and can then be peeled off the surface to which it adheres. Later, it begins to loosen and may separate spon taneously. When uncovered the mucous membrane may be found to be reddened and thickened, and if the inflammation has been severe, raw-look ing or even ulcerated.
On examination of the false membrane, it is found to present to the naked eye the appearance of coagulated fibrine ; but under the microscope is seen to consist of proliferated epithelial cells which are fused together into a network. These cells are cloudy from a peculiar degeneration of their protoplasm. A vertical section of the layer shows the undermost cells to be much smaller than those at the surface, and in a far less advanced stage of degeneration. Minute extravasations of blood are also scattered through the substance of the layer. If the vertical section be made in situ, and be carried clown through the mucous membrane, it will be seen that the exuded layer is seated directly upon the basement membrane, taking the place of the ordinary epithelial coating. When the morbid process comes to an end, degeneration Ceases ; a little purulent matter, formed by unaltered new cells mixed with serum, appears between the mucous surface and the false membrane covering it, and the latter is detached.