Microscopic experiments daily demonstrate the existence of myriads of animalcules in every substance. They have recently been discovered in the progress of certain crystallizations; and some philosophers maintain that most inorganic bodies are formed of the remains of organic substances. The existence of animalcules in the generative secretion was first noticed by Lewis Hamme, a young German student, and shown by him to Leeuwenhoeck, who published an account of them. Hartzoeken wrote upon the subject the following year, and asserted that he had seen these animalcules three years before they had been observed by Hamme. This curious subject soon attracted the notice, not only of physiologists, but of priests, artists, and even courtiers, for we find our Charles II. making curious inquiries on this investigation. Although many opticians could not discover these creatures, the eyes of courtiers were more keen than theirs, and to gratify their royal master’s depravity, described them most minutely. Their length was ³/100000 of an inch, their bulk such as to admit the existence of 216,000 in a sphere whose diameter was the breadth of a hair, and their rate of travelling nine inches in the hour. They saw them in the seminal secretion of every animal; and, what was still more remarkable, they were of a similar size whatever might have been that of the animal: they saw them in the sprat and in the whale; they could distinguish the male from the female; and they all moved along in gregarious harmony like a flock of sheep: nay, more; Dalenpatius actually saw one of them, more impatient than his companions, burst from his ignoble shackles, and actually assume the human form. At other times they were discovered swimming in shoals to given points, turning back, separating, meeting again, and frisking about like golden fish in a pond. Kauw, Boerhaave, Maupertius, Lieuland, Ledermuller, Monro, Nicholas, Haller, and indeed most of the philosophers of Europe, were convinced of their existence.

Buffon, however, and other naturalists, contended that these were not animalcules, but organic particles; and LinnÆus imagined them to be inert molecules, thrown into agitation by the warmth of the fluid. Finally, to determine the question, Spallanzani began an assiduous course of observations and experiments. He found these animalcules in the human species to be of an oval form, with a tail tapering to a point. This appendage, by moving from side to side, propelled them forward. They were in constant motion in every direction. In about twenty-three minutes their movements became more languid, and in two or three hours they generally died. The duration of their life, however seemed to depend, in a great measure, on the temperature of the medium: at 2° (Reaumur) they died in three quarters of an hour; while at 7° they lived two hours, and at 12½° three hours and three quarters. If the cold was not too intense, they recovered upon the temperature being raised; when only 3° or 4°, they recovered after a lethargy of fourteen hours; and according to the less intensity of the cold, they might be made to pass from the torpid to the active state more frequently. They were destroyed by river, ice, snow, and rain water; by sulphur, tobacco, camphor, and electricity; even the air was injurious to them: in close vessels their life was prolonged to some days, and their movements were not constant and hurried. They were of various sizes, and perfectly distinct from all species of animalcules found in vegetable infusions, &c. In short, Spallanzani completely confirmed the principal observations of Leeuwenhoeck, and satisfactorily explained the sources of the inaccuracies of other inquirers. Prevost and Dumas have recently confirmed the observations of the Italian physiologist.

This doctrine of life being perpetuated by the transmission of animated particles, or animalcules, is by no means of modern date. We find this theory advanced by Hippocrates, and Aristotle, and Plato. Democritus described worms that assumed, in the progress of their development, the human form; and Lactantius thus refuted his ideas: “Erravit ergo Democritus, qui vermiculorum modo putavit homines effusos esse de terrÂ, nullo auctore, nullÂque ratione.” Hippocrates plainly says, that the seminal secretion was full of animalcules, whose several parts were developed, and grew afresh; that nothing did exist that had not pre-existed; and that what we term birth was nothing more than that transition of these hitherto imperceptible animalcules from darkness to light.

Gesner has endeavoured to prove that the word ????, so frequently found in the writings of Hippocrates, and translated anima, was synonymous with insectum, animalculum, papilio. Plato, when expressing himself on this curious subject, compares the matrix to a fertile field, in which animalcules are gradually developed, at first of such a small size that they are imperceptible, but, by taking the food prepared for them, grow in strength until they are brought to light in a state of perfect generation; and St. Augustine thus follows: “Hunc perfectionis modum sic habent omnes ut cum illo concipiantur atque nascuntur; sed habent in ratione, non in mole, sicut ipsa jam membra omnia sunt latenter in semine; cÙm etiam natis nonnulla desint, sicut dentes, ac si quid ejusmodi.” In the works of Seneca we also find the same notions: “In semine omnis futuri hominis ratio comprehensa est, et legem barbÆ et canorum nondum natus infans habet; totius enim corporis, et sequentis Ætatis, in parvo occultoque lineamenta sunt.”

It may be said that these opinions were similar to those of the Ovarians, who, as we have observed already, believed that every thing arose from the egg. Such were Aristotle, Empedocles, and other philosophers: “For the egg is the conception,” said the first of these great men, “and after the same manner the animal is created;” but there was a manifest difference in their systems. Harvey, Haller, De Graef, were amongst the most warm advocates of this doctrine, which indeed prevails to the present day, as it would be difficult to find organized beings that did not spring from an original germ.

It thus appears that, notwithstanding the absurd doctrines of generation being founded upon the existence of these animalcules, they clearly do exist. Modern microscopic experiments daily confirm the fact; not only in the generative secretion, but in the other fluids of the body: creatures of an inch to an inch and a quarter in length have been found to inhabit the mesenteric arteries of asses and horses. Mr. Hodgson found them in seven asses out of nine. They have also been found in the blood of female frogs, salamanders, and tadpoles. What wonders are perhaps in store for the microscopic observer and the physiologist! All living matter seems to be animated by particles, by atoms, equally possessed of life. Does the vitality of these constituent molecules hold any influence over our existence? Is their life necessary to the preservation of ours? Is any agency destructive to them injurious or destructive to us? In a former paper I have recorded recent observations, where animalcules of a peculiar description were found in the purulent secretion attending various affections. A morbid condition seems thus to produce a new series of animated beings, or this new series of living atoms perhaps have produced a morbid state. Many eruptive maladies are either caused by the presence of insects, or insects are subsequently developed in their pustules. Wichmann, and many other physicians, have maintained that the itch was produced by an insect of the genus acarus, or tick.

Latreille has given a minute description of this creature in his Genera crustaceorum et insectorum, and calls this offensive species the sarcoptes scabiei. LinnÆus classed it among the aptera, and termed it the acarus scabiei. This insect is nearly round, with eight legs; the four fore-legs terminated with a small head, the hind ones with a silky filament. The Arabian Avenzoar had long since observed them, and it was from his writings that Mouffet was induced to pursue the inquiry. Redi, an Italian physician, was the first propagator of this doctrine in modern times, and published, in 1685, a paper of Cestoni of Leghorn, who had frequently observed mendicants and galley-slaves extracting these insects from the pustules of itch with the point of a pin, in the same manner as chigoes are extracted from their cyst in the West Indies.

It was this communication of Cestoni that led to a further and more minute investigation. Curiosity was every where excited, and the most learned and intelligent naturalists and physicians, amongst whom we find the illustrious names of Borelli, Etmuller, Mead, Pringle, Pallas, Bonani, LinnÆus, Morgagni, strove with incessant diligence to ascertain this important fact, which certainly was likely to shed a new light on our pathological speculations. The existence of the acarus was established.

The most conclusive experiments on the subject were those of GalÈs, in 1812. The following is the account of them: “I placed under a microscope a watch-glass with a drop of distilled water, after having carefully ascertained that it did not contain any visible animalcules. I then extracted from an itch pustule a small portion of the virus, which I diluted in the water with the point of a lancet. I watched most attentively for upwards of ten minutes, without having been able to notice any animation. Two similar experiments were equally ineffectual. Disappointed in my expectations, I was about giving up the task, when an idea struck me of submitting the liquid of the first experiment to another trial. I had left it in the watch-glass, exposed to solar heat. I then was not a little surprised when I discovered a perfect insect struggling with its legs to extricate itself from the viscid fluid that confined it. Having succeeded in reaching a more limpid part of the liquor, its form was so distinct that Mr. Patrix, who was with me, was enabled to take an exact drawing of its configuration.”

This curious result naturally induced GalÈs to pursue his inquiries, and he discovered that this insect chiefly occupies the pustules that are filled with a thin serum, and avoids those that contain a thicker secretion. Hence the watery pimples in itch are invariably those that produce the most intolerable prurience.

The next important question was to decide whether this insect was the cause of the disgusting disorder. For this purpose GalÈs placed several of them on the back of his hand. He then covered the part with a small watch-glass, kept in place with a bandage. Three hours after he awoke, experiencing a sensation of itching on the part. The following morning three itch pustules were evident, and convinced him that he had succeeded in inoculating himself with the loathsome complaint. This fact he communicated to Olivier, DumÉril, Latreille, and Richerand. Experiments in the hospital were immediately directed to be made, and all produced a similar result; affording a convincing proof that these insects could produce the affection, which they had merely been thought to have complicated.

Many writers, who, like Mason Good, had decided that “whenever these insects appear, they are not a cause but a consequence of the disease,” opposed and contradicted the statement of GalÈs, and the numerous practitioners who had procured and witnessed facts, which are never “stubborn things” to speculative minds. These writers maintained that whenever any organ was weakened, or in a morbid condition, it was apt to become a nidus for some insects or worms to burrow in. Hence the numerous varieties of invermination in debility of the digestive organs. But it is needless to observe that their objections cannot stand against the imbodied evidence brought forward in proof of their error. Bosc, Huzard, Latreille, DumÉril, and many other naturalists, subsequently found these acari in the eruptive diseases of many animals.

I repeat it, this subject is replete with interest; and microscopic experiments may some time or other throw a material light on the practice of medicine. Those substances that are known to destroy the insect that produces the itch, cure the malady. May not this analogy lead to singular results?