SECTION II.. The Animalcular Theory of Epidemics Untenable.

THE ANIMALCULAR THEORY OF EPIDEMICS UNTENABLE.

The animalcular theory of disease, after remaining almost unnoticed for nearly two centuries, has been again revived under the auspices of Dr. Holland in this country, and Henle of Berlin. And though not entirely buried in obscurity, this theory had completely failed to modify the practice of physicians in the treatment of those diseases which were supposed to owe their existence to these invisible atoms of created being. The resuscitated notions and all their amplifications, to which the advance of science has contributed so much, are threatened with a like fate, an absence of all practical results.

Though I would not attempt to deny the possibility, nay, even the probability, that insect life may yet be discovered as the cause of some diseases, still {129} there are many and cogent reasons against both, and which are at variance with facts and observations. Where insect life has been found associated with disease, it more especially appears as a consequence than as a cause.

Disease, in its most enlarged sense, is a conversion of one form of matter into another; it is a transformation of healthy blood and tissue into new and abnormal products. Where insects in all their variety of forms are discovered, their voracious propensities are their chief characteristics, they are the consumers of matter after its partial disintegration, if animal matter be their food, unless they be carnivorous and predacious, or if herbivorous they usually feed upon the tender shoots of plants. Thus far we are certain of the manner in which insects destroy living matter; it is a process the unassisted eye may every where witness, and which experience has amply attested. To take, however, the animalcular world as it presents itself to us under the microscope, and as the intermediate step between the manifest and the hidden for a fairer and more direct method of reaching the truth, what do we observe to be the ruling law of infusory instinct? They live to feed; the term polygastrica sufficiently implies their natural tendency to consume. The simplest form of animalcular life, seen in the genera of monads, still preserves the animal character by possessing a stomach or stomachs in which the food is received, to be digested for the nourishment of the {130} system; and even some of these minute objects which vary in size from one two-thousandth, to one three-thousandth of a line in diameter, are said to be carnivorous and predacious. Upon this fact alone, I would place the improbability of insects being the cause of epidemic disease. Each insect doubtless has its own peculiar food, and whether it be a vegetable or animal feeder, it consumes the matter already organized for conversion into its own tissue, and the only change which could be affected by them in the blood, would necessarily be that of appropriation of some one of the constituents as an element of food; when that food is digested, (taking digestion generally as an identical process,) the excrementitious matter is composed of secretions and disorganized matter, mixed together as an effete product, and destined then for reorganization by the vegetable kingdom. Now all animals, whether they be large or small, live on organized matter,--they convert that matter into an inorganic form, and I cannot help imagining that if epidemic diseases and fevers depended upon animalcular growth and development in the blood or tissues of the body, the excretions or secretions from them would have yielded some information to the searching enquiries of the chemist, supposing that these excretions and secretions were capable of reaching to a sufficient amount in quantity, to bring about those fatal effects of poisoning, we witness in Cholera and other epidemic affections. Insects, I {131} believe are poisonous only by their secretions, and though they are known to multiply with exceeding rapidity, I can hardly imagine that by their development, however rapid, they could produce such a change in the human body, as to bring about the speedy dissolution, and generally gangrenous appearance, that has invariably been observed in those suddenly dying under the influence of epidemic poisons. The vibriones, whose destructive effects on wheat are so well known, are a genus of animalcules, which at first would seem to favour the animalcular theory in a remarkable manner; for on examining them, they do not appear to possess any other structure than a gelatinous absorbing mass, in this respect resembling a vegetable.

But Ehrenberg's scrutiny corrected the error of De Blanville, and shewed, that they were far from being agastria, or stomachless animals. The Rev. William Kirby says, "Ehrenberg has studied the vibriones in almost every climate, and has discovered, by keeping them in coloured waters, that they are not the simple animals that Lamarck and others supposed, and that almost all have a mouth and digestive organs, and that numbers of them have many stomachs." All the discoveries indeed which have been made on the minuter forms of animal life, have tended to confirm the doctrine that the stomach is the exponent organ of an animal; that is, in all animals there exists, in a variety of modified conditions, a receptacle for food. Some of the {132} animalcules, however, are still supposed to exist by absorption, as the vinegar eel, vibrio anguilla, but when we find that the law is, generally speaking, that the receptacles of food become multiplied in number in these minute beings, and the vibriones which were supposed to be stomachless, have been proved to emulate their associates in the number of these organs; it would be more reasonable to conclude that our imperfect vision is the barrier to their detection, rather than to suppose that they do not exist. Besides, when we are told on undoubted authority that some of the animals of this class, have as many as forty or fifty stomachs; the least we can do, is to allow that all of them possess, at least one digestive organ, though we may not be able to detect it.

So far then for the consideration of animalcular structure: let us now more particularly enquire into their destructive habits, and their functions, inasmuch {133} as they may be supposed capable of engendering epidemic diseases and fever. The truly carnivorous animalcules, or those truly herbivorous in their instincts, we may presume to be beyond the limits of our enquiry. We have rather to do with those which take an intermediate position, namely, those which feed upon matter undergoing decomposition, or upon fluids containing organic matters in solution, or suspension. If we take Entozoa generally, they may be considered as most conveniently to be placed in this intermediate class; and here we find still the digestive apparatus, and more than this,--for upon the modifications of the organs appropriated to digestion is their classification founded. "Rudolphi divided the Entozoa into Sterelmintha, or those in which the nutrient tubes without anal outlet are simply excavated in the general parenchyma, and into the Coelelmintha, in which an intestinal canal with proper parietes floats in a distinct abdominal cavity, and has a separate outlet for the excrements."

How do these animals obtain their sustenance, and what changes can they produce upon the vital fluid of the body? Analogy is here our only guide. If the trichina spiralis is examined, it is found to be enclosed in a cyst containing fluid; and this is, {134} doubtless, the source of its nutriment, and contains in solution the elements for its nutrition; but in this instance there is no selection, and there can be no locomotion to an extent sufficient to imply searching for food, as the animalcule in its natural state, when taken from the human muscle, is found coiled upon itself, making about two and a half turns. The fluid of the cyst is thus in all likelihood prepared by endosmosis, for the immediate and appropriate nutrition of the parasite. The cyst is thus the part which performs the diseased process, the containing animalcule is merely the consumer of what is prepared for it by the cyst. And this would seem to be the rule with all parasites, of the encysted kind.

We have alluded to the vibriones which are found in the fluids of living bodies, and the trichina which is found in the solid muscle; we have now to refer to those which infest the cavities. It was, I believe, Ehrenberg, who shewed that the tartar which accumulates on the teeth is composed of the debris of minute animalcules; in fact, that it consists of calcareous matter, having once formed a portion of the structure of their bodies, the ubiquity of these creatures is therefore as much and clearly established as the lower forms of vegetation. The intestinal worms, of which perhaps the Taenia is the most curious and important to be noticed, are from the locality in which they are found, chiefly injurious by the irritation they set up, and by appropriating {135} to themselves the nutrient juices elaborated in the process of animal digestion, thus depriving the individuals they infest of that which was destined for their own nourishment. In this, as in all associated instances, the character by which these parasitic animals are marked is their consuming propensity. There is, however, one more observation to make upon parasitic growths; but the question is yet unsettled in what kingdom of nature is the acephalocyst, or hydatid, to be placed. Mr. Owen says, "As the best observers agree in stating, that the acephalocyst is impassive under the application of stimuli of any kind, and manifests no contractile power, either partial or general, save such as results from elasticity, in short, neither feels nor moves, it cannot, as the animal kingdom is at present characterized, be referred to that division of organic nature."

We thus arrive at the simple cell, and the multiplication of living beings by cell buds; it is the point at which the confines of the animal kingdom are reached, and at which we are driven to speculation. The hydatid lives like a plant, by imbibition; and procreates, like a plant, by budding, either endogenously or exogenously, as regards the original or parent cell.

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This condition of being, suggested the notion of Protozoa, or first animals, in the same way that the purely cellular plants, that is, each individual, consisting of a single cell, gave the idea of Protophyta, or first plants. Mr. Kirby thus expresses himself on this subject: "The first plants, and the first animals, are scarcely more than animated molecules, and appear analogues of each other; and those above them in each kingdom represent jointed fibrils."

Admitting, then, that animals as well as plants exist in the form of simple cells, and that their multiplication proceeds apparently upon the same principle in each, it is nevertheless abundantly manifest, that the cellular form of perfect individuals is infinitely more numerous in the vegetable than in the animal kingdom.

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From the mosses downwards to the fungi, the whole structure of the plants consists of an aggregation of cells, more or less in number and complicate arrangement, until, through a variety of gradations, we reach the single cell as a perfect individual.

It is rather remarkable, that the lower forms of vegetables and animals seem to derive their nutriment from matter of a similar kind; and though the office of plants is as a rule, to convert inorganic into organized matter, it appears that some of the fungi may live as animals do on organic matter when in a state of solution. This, however, is uncertain; for we do not know what are the first signs of decomposition in organized bodies, and for aught we can tell, it may be perpetually going on; so far as the disengagement of carbon from the system is concerned, this is certain; but whether the nitrogenous compounds also are subject to a resolution into their elements in the living body, is another question, and not so easy of solution. The partially decomposed elements of animal structures are, however, particularly adapted for the nutrition of the lower forms of vegetation; it is, indeed, from the decaying organic matters that the fungi derive, it may be said, their entire food.

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