Another irregularity of the pock is what is familiarly termed the raspberry excrescence. A red elevation forms at the seat of inoculation, and at first promises to follow the typical course, although it may be tardy in making it appearance; but it never advances to full development. It becomes indolent, and may last for several weeks, or even months, in the form of a hard, flat nodule of a bright-red color, not unlike a small nævus. In many instances it has a succulent look, but no lymph can be obtained on puncturing it. No areola appears at any time, and finally the lesion slowly disappears, leaving no trace of its existence. It is probably an abortive form of pock, in which only the papillary layer of the skin takes part, without any exudation into the epidermis. It is seldom, if ever, protective against small-pox, for it constitutes no bar to a subsequent vaccination. This irregular pock has been observed from time to time ever since the early days of vaccination, but for the past six years it has been seen more frequently in New York than for many years before. Now, however, it seems to be growing less common. The writer is not aware of any satisfactory explanation of its occurrence. It is seen in all sorts of subjects, and seems to follow the use of one variety of virus as much as the employment of any other.
What has been termed generalized vaccinia is another form of irregularity. The expression is a vague one, covering as it does not only the very rare cases of true eruptive vaccinia, in which a general eruption of pocks takes place as a consequence of constitutional infection, playing the part of an exanthem, but in addition those instances, not very uncommon, in which pocks are formed here and there on the body, probably as the result of the accidental transfer of the virus from the pock by scratching. Under such favorable conditions—the immediate transfer of lymph from a pock in which the specific evolution is going on vigorously—the slightest penetration of the epidermis with the nails is enough to secure self-inoculation. In view of this facility with which it may be effected, we should be very careful not to jump hastily to the conclusion that in any given case of generalized vaccinia the supplementary pocks are truly eruptive; as a matter of fact, the present writer has never seen an instance in which he was convinced that such was the case. Where the pocks are very numerous, especially in subjects with an irritable skin, much distress may be caused by the itching and by the consequences of scratching, and marked febrile reaction may accompany the process; so that, in view of the great similarity of the lesions to those of the variolous eruption, much doubt is sometimes entertained as to whether the disease is not really small-pox. This question cannot always be definitely settled at first, but the failure of the secondary fever of small-pox, together with the fact that the disease does not spread by infection, will generally suffice to decide it.
Concerning those cases of generalized vaccinia that are manifestly not eruptive, it sometimes happens that the cutaneous receptivity is not exhausted for several weeks, or even months. Such cases set Bryce's test at defiance, in consequence, probably, of an idiosyncrasy. In some of these instances the pocks appear in clusters of successive formation, looking not unlike patches of zoster. Small supplementary pocks in the immediate neighborhood of the original lesion are not at all uncommon.
PATHOLOGICAL ANATOMY.—Avoiding the minute histological details for which the prescribed length of this article gives no scope, but little is to be added to what has already been said in the section on the clinical features of the disease. The lesions of vaccinia are wholly cutaneous. Confining ourselves to cases that follow a regular course, there is, indeed, but one, the pock—a term that seems preferable to vesicle and pustule, since the latter apply only during certain phases in the development of the lesion.
A pock may be regarded as essentially a lesion of the epidermis, for it is in that structure that its most striking features are developed, and in some cases, although doubtless the papillary layer of the derma is congested, there is no permanent alteration of tissue below the Malpighian layer of the epidermis. These are the catarrhal pocks of Rindfleisch, and it is in such cases, if in any, that no scar (even of temporary duration) results. The term catarrhal pock, however, is not vitiated by an extension of the morbid process deep enough to produce a permanent cicatrix, and it is probable that in most cases the catarrhal type predominates. By the term diphtheritic pock the same author refers to cases in which the congestion of the papillary layer is so intense as to block the supply of blood to the apices of the papillæ, as a result of which they become exsanguinated and necrosed, forming a white pultaceous layer on the floor of the pock, which is undoubtedly what Ceely referred to when he spoke of a false membrane. In some cases even the subcutaneous tissue undergoes necrosis, a sort of core being included in the substance of the crust that ultimately forms.
Whichever of these forms of pock we take into consideration, always excluding irregularities and complications, we find certain definite changes in the epidermis. The dome of the pock is formed by the unbroken transparent horny layer of the epidermis, unaffected by the morbid process. The cavity of the pock is formed by the squamous cells of the epidermis being forced out of their normal relations by an exudation of lymph between them, some of them being tilted up edgewise while still retaining their connection with the surrounding cells, thus accounting for the multilocular structure of the pock; for it is a fact that the circular bourrelet consists not of one ring-like cavity, but of many separate chambers. The result of this structure is, that the liquid contained within the pock—the lymph—escapes only partly through a puncture made in the wall of the vesicle. In order to evacuate the pock thoroughly it is necessary to make a great number of punctures or a circular incision following the ring-like ridge of the bourrelet.
The lymph contained within the cells of the pock is a liquid which in its gross physical properties differs but little from the lymph which exudes from any traumatic surface shortly after the injury has been inflicted, as in the glazing process that takes place in wounds. Examined microscopically, however, it is found to contain not only the fibrin, the salts, the corpuscular elements, and the débris that ordinary tissue-juice presents, but also certain minute spherical bodies—termed microspheres, microzymes, vaccinads, etc.—that give it its characteristic infective quality and justify the title of virus commonly applied to it. That these minute bodies really constitute the virulent element of the lymph, or at least that they are the vehicle of the contagium, is not a mere matter of conjecture, but has been demonstrated abundantly, notably by Chauveau and Sanderson's diffusion experiments. Inoculation with the supernatant liquid, containing none of these bodies, always fails to convey the disease, but it is not absolutely essential that they should be present in large proportion in the lymph to render the latter virulent, for Chauveau found that lymph diluted with thirty times its bulk of water was not without infective power. It scarcely need be said, however, that the greater the proportion in which they are present, the greater is the probability that the lymph will prove infective on inoculation. These bodies have been supposed to be of a vegetable nature, and Hallier, Kohn, and others have bestowed no little study upon their botanical characteristics. Under favorable circumstances they retain their virulent properties for a long time, especially if kept perfectly dry and not subjected to a high temperature. The present writer has met with success in the use of vaccinal virus seven years old.
The lymph differs somewhat in its gross appearances according as it is produced in man or in the bovine animal. In the former it is clear and limpid, and exudes freely in great drops when the pock is punctured in its peripheral portion; in the latter it is more straw-colored and more viscid, exuding sluggishly, or even refusing to flow without the aid of pressure. Moreover, the vaccinads seem endowed with different properties in the two cases: in man they have a tendency to remain equably diffused through the liquid, while in the cow they tend to separate from it and to be deposited upon any solid surface at hand.
The phenomenon termed umbilication, common to the vaccinal pock and to that of variola, has given rise to some differences of opinion as to the mechanism of its production. The term implies a depression at the centre of the pock. This appearance is not invariable, but it is constant enough to have met with general acceptance as a characteristic feature, notwithstanding the undoubted fact that it is found in lesions that have nothing whatever to do with any of the varioliform diseases. Not to waste space in discussing the various theories that have found supporters, it may be said that they have all been proved to be defective, save only the simple explanation that as the process of evolution advances the centre of the pock undergoes desiccation, whereby that portion of the tissue involved is so glued and drawn together as to become incapable of the swelling that is still going on in the growing peripheral portion of the lesion.
The crust into which the pock ultimately becomes converted is not, as is commonly supposed, mere dried lymph and nothing else; it is dried tissue enclosing concrete lymph. It generally includes also various sorts of débris—broken-down epithelium, blood-corpuscles, pus-corpuscles, and even, in rare cases, a core of sphacelated tissue like that of a furuncle.