The following questions will naturally occur to many of my readers. What gives rise to these tubercles? what are the predisposing causes which lead to their formation? and, when formed, how do these apparently unirritating bodies produce effects so baneful? Queries like these, however, cannot shortly be replied to, leading, as they do, to discussions which embrace many curious theories; but as the negative mode of teaching is often of avail where the positive or more direct would fail to bring conviction, I shall, before proceeding to allude to what the causes are, endeavour to state what they are not.
Imaginary Causes of Rot. The liver-fluke has long been looked upon as the origin of rot, and this opinion has now become so deeply rooted, and taken so fast a hold of the public mind, that if I were to contradict it by plain assertion, I should only be striving to buffet singly a tide of opposition. The best way, therefore, will be to examine a few of the theories supposed to be confirmatory of the notion that fluke worms are the beginning of the mischief, and then see whether their supporters have managed to make good the point.
I. The fluke is supposed to get into the liver of the sheep by being swallowed, and this, according to our theorists, may be brought about in some of the following ways:—
1. The eggs may be floating in the air, and thus accidentally reach their destination. This is the view taken by the celebrated Clater; but if he had been, in this instance, a man of experiment, rather than of idle conjecture, he would have found, as any one readily may, that the eggs of the fluke worm sink in water, and, consequently, that they cannot float in air.[ [35]
2. The Rev. Dr Singer, to whom Scotland at large, and Dumfries-shire in particular, is much indebted for numerous and valuable papers on agricultural subjects, states, in the third volume of the Highland Society's Transactions, page 478, that "The spawn or eggs of the liver fluke are most probably conveyed upon the grass by summer watering, and afterwards taken into the stomach with it." A few lines further on, he speaks of the eggs being "wafted thither by harvest waterings." Now, as the fluke is only produced within the sheep, I need only put the unanswerable questions—How are they conveyed to the grass? and from whence are they wafted? to refute at once this hasty notion.
3. The eggs may be voided by the sheep, may fall upon the herbage, and there remain till they are eaten. Such is the supposition published by Mr King of Hammersmith, in the Quarterly Journal of Agriculture, No. XXXI. p. 331, in which, after showing the vast number of eggs which must fall upon the grass, he says, "We must cease to wonder that so many sheep die of rot; the miracle is, that every sheep does not die of it."! I cannot, however, for my part, see a miracle in the matter, for the simple reason, that the eggs of the entozoa are not capable of retaining their vitality when absent even for a very short time from the place of their nativity, and therefore may be eaten with impunity.
II. Supposing the eggs to have reached the sheep's stomach in a condition to allow of their being hatched, they, according to popular voice, find their way into the gall bladder by one of two routes.
1. Mr King, the gentleman above spoken of, conjectures, in the same paper, that the fluke, after leaving the egg in the stomach of the sheep, makes its way up the gall vessels. This is, I am sorry to say, a very idle conjecture, as, from the valvular nature of the opening of the gall duct into the duodenum, an entrance from that intestine to the gall bladder is perfectly impracticable to any of the entozoa.[ [36]
2. The eggs are believed by a writer in the Letters of the Bath Society of Agriculture, for 1781, to be taken into the blood along with the chyle from the small intestines, and to be arrested in the liver by the secretory ducts. This, it must be clear to every one, is the most absurd of all the notions; for if a globule of blood, which we must suppose to be the largest body capable of being absorbed from the intestine, is only about 1/3000 of an inch in diameter, how can the egg of a fluke worm pass through the same channel, when Mr King has, by careful observation, shown it to be 1/300 an inch in its shortest measurement. Again, allowing that they are taken into the blood, would they not frequently be hatched there, and would they not also be found in other quarters besides the liver. But do we ever find them in the blood? Do we ever see them in other organs? Certainly not.
Not one of these theories would ever have been broached had their authors been aware of two important circumstances. 1. That M. Schreiber, the director of the Museum at Vienna, has proved that worms and their ova are not capable, under ordinary circumstances, of resisting the action of the digestive organs, and, therefore, that they cannot be introduced into the body by this channel. "During six months, he fed a pole-cat almost exclusively on various kinds of intestinal worms, and their eggs mixed up with milk; and on killing and examining it, at the end of this period, not a single worm of any kind was found in it."[ [37] The reader may perhaps object to this illustration, on the ground that there is so vast a difference between a sheep and a pole-cat, that a comparison in regard to their digestive habits cannot possibly hold good, but if he will turn to paragraph (96), he will see that the stomach of a sheep is as well fitted as that of a carnivorous quadruped for the digestion of animal matters. 2dly, the fluke worm has been found by Frommen in the fœtus of the sheep, into which it could not have been conveyed by transmission from the mother, as there is no direct vascular communication between the fœtal and maternal side.