(161.) Appearances on Dissection. The first thing which strikes a person on viewing the carcass of a sheep which has died of rot, is its leanness. In conducting the examination, the fell appears of a bluish white, the muscles are pale and wasted, and fat hardly to be met with. Where it once existed, a tough yellow substance alone remains, which is so destitute of all pretentions to the name of suet, that it cannot, even when thrown upon a fire, be made to blaze. Dropsical accumulations are found in the legs, chest, neck, and belly. On opening the chest, the lungs are often seen adhering at intervals to the lining membrane of the ribs, and have always a shrunk, diminutive appearance. These adhesions are frequently seen where there are no tubercles, and are in that case simply the result of exposure to cold; but where they are coincident with tubercles, they may be ascribed either to the animal having been exposed to cold, or to the inflammatory action set up by the tubercles themselves. The lungs are always the principal, and I may also, from my own experience, add, the primary seat of the affection. When examined in the early stage of rot, they have a hard lumpy feel, especially at the upper part or lobe, and at this time a great number of irregular yellowish white, patchy-looking bodies (Plate VI. fig. 2.), will be seen shining through the membrane, pleura, which surrounds the organ. These tubercles, as the hard white bodies are called, vary in size from that of a mustard seed to that of a pea. They are sprinkled through all parts of the lung, and will in every dissection, be found in a variety of stages, from the firm condition in which they were deposited, to the softened state which denotes their speedy expectoration. Each tubercle, however small, usually holds a particle of calcareous matter in its centre. The lungs, in the advanced stage of rot, will be full of cells or caverns, owing to the destruction of its texture by suppuration in those parts where tubercles existed. The cells or sacs are of all sizes, from that of a bean to that of a goose egg; but if the animal has been taken care of during the progress of the complaint, and lingered long, the abscesses will be so numerous, and so closely situated, as to give the remains of the lung the appearance of a large bag. Extreme cases of this nature are, however, rare; as the sheep, in general, either falls before the knife, or is killed from exposure to cold long ere the disease has reached its farthest limit. These sacs contain purulent matter, of all shades and odours, and identical with that which the animal coughed up.
Tubercles, and all their concomitants as above detailed, are also met with in the liver, though not so frequently as in the lungs. They constantly occur in the clyars (mesenteric or lacteal glands) which on this account are much above their usual size, and are occasionally found in other parts; but I need not proceed in their description, as sufficient has been said about them to enable the unprofessional reader to understand their relation to the complaints.
Fluke worms and hydatids are almost constant attendants on rot, and seemingly most important ones, especially the former, which have, I may say, kept a great bulk of the learned and unlearned for many years in a perpetual bustle, and have so effectually hoodwinked writers on this subject, as to prevent them seeing the truly important points in the disease. For this reason, I hold them worthy of particular description; as it is only by becoming acquainted with their history and habits that we can form correct ideas either of their mode of origin or of their supposed ability to cause rot.
(162.) The Liver Fluke (fasciola hepatica or distoma hepaticum, Fig. 6. Plate I.) derives its name from the resemblance it bears to the plaice or flounder, though its shape has been more aptly compared to that of a melon seed. It is flat and oval, of a brownish-yellow colour, and varies in size from that of a pin-head to one inch in length, and half an inch in breadth. Each worm is bisexual or hermaphrodite, on which account they multiply with great rapidity. The generic name—distoma—signifies having two pores, a.b. Fig. 6, Plate I., and is for this reason applied to it. The nipple-like body at the extremity a. contains the orifice of the pore or opening leading to the female division of the generative apparatus, situated between a. and b. In this cavity are formed the eggs, which are at intervals protruded, to be hatched when floating in the sheep's bile. The male organ is situated in front of the ventral pore. The anterior opening b. is equivalent to a mouth, and leads inwards and backwards, communicating with the intestinal canals c., which are easily made out in the recent animal, from their containing dark bile, and which, as in other creatures low in the scale of being, serve the double purpose of a digestive and circulatory apparatus; that is to say, the stomach first prepares a fluid which is equivalent to blood, and then distributes it throughout the body.[ [33]
Flukes are never found in the arteries of the liver, as has been erroneously stated by some writers, their abode being limited to the gall bladder and its ducts. In these they are often present in such numbers as to cause great distention of the sac and tubes, and in some instances the irritation produced by them leads to the thickening of the walls of the gall bladder, and to a deposition of calcareous matter between its coats; frequently also to complete obliteration of portions of the ducts. Hence the crackling sound sometimes perceived when handling the liver of a rotten sheep.
(163.) The Hydatid or Blob (Cysticercus tumicollis, Fig. 1. Plate VII.) so frequently found in sheep, is in form one of the simplest of the entozoa (literally dwellers within), being little more than a bag containing a quantity of fluid. As relates to outline, this hydatid bears no small resemblance to a Florence flask. It is said to have a head, h.—a neck, n.—a body, b.— and a posterior or caudal vesicle, c.v. Its claims to the title of an animal have been much disputed, but as it has been seen to move spontaneously, and as the contained fluid is always essentially different from that by which the hydatid is surrounded, the question may be looked upon as set at rest.[ [34]
The method of their reproduction is in unison with their structure, simple in the extreme. Nothing, however, very precise is known about the process. The vesicle which acts as heart and stomach serves also as the reproductive cavity, but how or by what means it is fecundated would be difficult to determine. The young hydatids are found adhering to the inner surface of the parent cavity. When they have attained maturity, the parent dies and shrivels, and the young ones begin to eliminate their nourishment from the juices of the quadruped which they infest.
These entozoa are found in general on the surface of the intestines, between them and their outer membrane (peritoneum), and on the exterior of the lungs and liver. They are always included in a cyst, to the inner surface of which they adhere by means of two hook-shaped processes projecting from the head. These cysts are always on the surface of the different viscera of the sheep, and in this way may be distinguished from another sacular animal, or rather supposed animal, termed acephalocyst or headless bag, which is sometimes found in clusters in the substance of the lungs, liver, &c. and is often confounded with the true hydatid.
(164.) Causes of Rot. If any one had been asked, thirty years ago, the cause or rather causes of the appearances which pass under the name of rot, he could not have enumerated them even in a day, for at that time each symptom was a disease, and as such was reckoned worthy of a separate and proximate cause. Nor could any person have had the courage to promulgate a common-sense opinion on the subject; for simple views regarding the diseases of domestic animals were then either not deemed worthy of a moment's notice, or, if considered, were swept at once, by the strong current of prejudice, into the foul ocean of predetermined disapproval. Opinions in cattle medicine were at that time valued according to the prolixity of their detail; and the more improbable the dependence of the effects upon the cause assigned, so much more was its discoverer lauded, and in like proportion was the chimerical fabric he had raised admired. Times are, however, now happily changed; that potent oculist, the march of intellect, has cleared the film from the public eye, and no one need, at present, be afraid to state the unaspiring fact, that tubercles are the sole and proximate cause of the disease called rot.
The observations of the late Dr Coventry, already quoted, would lead us to suppose that tubercles are of rare occurrence in the lungs of sheep, but in refutation of this assertion, I need only request the reader to take a ramble through a butcher-market, and he will perceive, even on cursory inspection, the fallacy of this conclusion. What the state of the liver is which is attended with flukes and hydatids, he has left us to make out. Chronic hepatitis, which accompanies tubercles in the liver, goes for nothing as a disease of sheep, and therefore does not require a notice; besides, it is not rot, and is quite incompetent of itself to cause it. As for the scurvy of which he speaks, he evidently means the disease now generally known by the name of Pining, but which, as it has no connexion with rot, and has only become prevalent within the last sixteen years, could not be very well known to him.