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]
[35] To obtain the eggs of the fluke worm for examination, hold a saucer under the gall bladder, make an opening in it with scissors, and the bile containing the eggs will flow into the dish. Pick out any fluke worms that may be in the fluid, then dilute it with about twelve times its bulk of water, agitate for a few minutes, and filter. The eggs will be found in the corner of the filtering paper.
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.