Fig. 377.—Monads in Rat’s Blood, stained with methyl violet, showing membrane under different aspects; blood-corpuscles, some crenated and others with stained discs (× 1,200).—(Crookshank.)
Professor Virchow draws the following conclusions:—“1. The ingestion of pig’s flesh, fresh or badly dressed, containing Trichinæ, is attended with the greatest danger, and may prove the proximate cause of death. 2. The Trichinæ maintain their living properties in decomposed flesh; they resist immersion in water for weeks together, and when encysted may, without injury to their vitality, be plunged in a sufficiently dilute solution of chromic acid for at least ten days. 3. On the contrary, they perish and are deprived of all noxious influence in ham which has been well smoked, kept a sufficient length of time, and then well boiled before it is consumed.”
A more minute Filarian worm has been detected in the human blood-vessels, known as Filaria sanguinis hominis. This worm carries on its work of destruction throughout the night; during the day it remains perfectly passive. It increases rapidly, and produces swellings of the glandular structures of the body, somewhat after the nature of those characteristic of the Bombay plague, with a slight difference, that after death the swellings are seen to be due to the vast accumulations of the Filaria sanguinis blocking the blood-vessels. The accompanying [Fig. 377] shows a similar infiltration of monads in the blood of rats dying of plague in Bombay.
Trematode Worms.—In the order Trematoda, to which the fluke belongs, the body is unsegmented, and to the naked eye smooth throughout, with a blood circulatory system, and two suctorial discs at the hinder end. There is a distinct digestive canal, usually forked, furnished with only one aperture, the mouth. The excretory organs open out as in tape worms, and the male and female organs co-exist in the same individual.
The Fluke (shown in [Plate IV]., No. 103) is cone-shaped, and is the Amphistome conicum of Rudolphi. This parasite is common in oxen, sheep, and deer, and it has also been found in the Dorcas antelope. It invariably takes up its abode in the first stomach, or rumen, attaching itself to the papillated folds of the mucous membrane. In the full-grown, adult stage, it rarely exceeds half an inch in length. It is certainly one of the most remarkable in form and organisation of any of the internal parasites.
The larger fluke (Fasciola hepatica) often attains to an inch or more in size. It is not only of frequent occurrence in all varieties of grazing cattle, but has likewise been found in the horse, the ass, and also in the hare and rabbit and other animals. Its occurrence in man has been recorded by more than one observer. The oral sucker forming the mouth leads to a short œsophagus, which very soon divides into two primary stomachal or intestinal trunks, the latter in their turn sending off branches; the whole together forming that attractive dendritic system of vessels so often compared to plant-venation. This remarkably-formed digestive apparatus is represented in [Plate IV]., Nos. 106 and 107, Fasciola gigantea of Cobbold, and should be contrasted with the somewhat similarly racemose character of the water-vascular system. Let it be expressly noted, however, that in the digestive system the majority of the tubes branch out in a direction obliquely downwards, whereas those of the vascular system slope obliquely upwards. A further comparison of the disposition of these two systems of structure, with the same systems figured and described as characteristic of the Amphistoma, will at once serve to demonstrate the important differences which subsist between the several members of the two genera, if we turn to the consideration of the habits of Fasciola hepatica, which, in so far as they relate to excitation of the liver disease in sheep, acquire the highest practical importance. Intelligent cattle-breeders, agriculturists, and veterinarians have all along observed that the rot, as this disease is commonly called, is particularly prevalent after long-continued wet weather, and more especially so if there have been a succession of wet seasons; and from this circumstance they have very naturally inferred that the humidity of the atmosphere, coupled with a moist condition of the soil, forms the sole cause of the malady. Co-ordinating with these facts, it has likewise been noticed that the flocks grazing in low pastures and marshy districts are much more liable to the invasion of this endemic disease than are those pasturing on higher and drier grounds; a noteworthy exception occurring in the case of those flocks feeding in the salt-water marshes on our eastern shores. [Plate IV]., No. 106, Fasciola gigantea: the anterior surface is exposed to display oral and ventral suckers, and the dendriform digestive apparatus injected with ultra-marine; No. 107 shows the dorsal aspect of the specimen and the multiramose character of the water-vascular system, the vessels being injected with vermilion.
In their larval condition the Amphistoma live in or upon the body of the pond-snail. This we infer from the circumstance that the larvæ, or cercariæ, of a closely-allied species, the Amphistoma subclavatum, are known to infest the alimentary canal of frogs and newts, and have also been found on the body of the Planorbis by myself. The cercariæ larvæ are taken, it is believed, by the sheep and the cattle while drinking. The earliest embryotic stage in which I have found the embryo fluke is represented at [Fig. 378], No. 1. In the year 1854, whilst observing the habits of Limnœa and other water-snails, I brought home specimens from the ornamental water in the Botanic Gardens; upon these were discovered thousands of minute thread-like worms, subsequently met with on other embryos, and at first taken to be simple infusorial animals, but upon placing them in a glass vessel these minute bodies were observed to detach themselves and commence a free-swimming existence. A fringe of cilia was seen to surround the flask-shaped body (No. 1).
Fig. 378.—Forms of Cercaria; stages in the development of the Fluke.