558 SHEEP HUSBANDRY. 



LOCAL DISEASES. 



" Grub in the Head." — If the " grubs " found in the fi-ontal and max- 

 illary sinuses of the sheep actually, in any case, produce disease, it must be, 

 in my judgment, by the irritation and inflammation which they induce in 

 the mucous membrane which lines those cavities. The popular theory 

 that the grub causes death by boring through the bony walls which sur- 

 round the brain, and attacking the substance of the brain itself, is, it seems 

 to me, utterly absurd. The only part of the skull where it could even be 

 fancied that such a perforation would be practicable, is the cribriform plate 

 of the ethmoid bone (11 of fig. 49,) which is very thin and is pierced with 

 numerous small holes for the passage of nerves. But an inspection of the 

 same figure will show that the sinus where the parasite is generally found 

 lodged, is not in immediate juxtaposition with the cribriform plate, and 

 that a passage from the former to the brain, would lead directly through 

 the frontal bone — the thickest one of the whole cranium, I never saw but 

 one gi'ub in the cells of the ethmoid bone near the cribriform plate, and 

 that, I judged at the time, was thrown there accidentally by the violence 

 attending the opening of the head.* But if the grub actually penetrates 

 to the brain, the fact would readily be disclosed after death. The full- 

 grown gi'ub would necessarily leave an orifice of considerable diameter 

 through the skull. Who has seen any such orifice in the cribriform plate 

 or elsewhere ] Who has seen any orifice but the natural ones of the crib- 

 riform p\site,JiUed with the nerves which pass through them ? The farmer 

 splits open the head of a sheep with an ax, cutting, mangling and scatter- 

 ing its contents, by the repeated blows necessary to eft'ect his purpose. — 

 Under such circumstances grubs are sometimes found scattered through 

 all the nasal cavities — over and among the brains — and on the ground. — 

 The proof is just as strong, here, that prior to opening the head, some of 

 the grubs were on the ground, as that they were in the brain ! 



The " grub " of popular parlance is the larva of the (Estrus ovis, or gad-fly 

 of the sheep. The latter is represented of the nat- 

 ural size in figures GO and 61. It is composed of Fig, 60- Fig. 61. 

 five rings. It is tiger-colored on the back and 

 belly, sprinkled with spots and patches of brown. 

 The wings are striped. The comparative propor- 

 tions of the head, corslet, wings, etc. are sufficient- 

 ly seen in the cuts. He who desires a full, scien- 

 tific description of these insects, or who would sheep gad-?ly. 

 fully investigate their habits and economy, will do 



well to consult the excellent monograph of them by Mr. Bracy Clark 

 the celebrated veterinarian. 



The sheep gad-fly is led by instinct to deposit its eggs within the nos- 

 trils of the sheep. Its attempts to do this, most common in July and Au- 

 gust, are always indicated by the sheep, which collect in close clumps 

 with their heads inward and their noses thrust close to the ground, and in- 

 to it, if any loose dirt or sand is within their reach. If the fly succeeds in 

 depositing its eg^, it is immediately hatched by the warmth and moisture 

 of the part, and the young grubs, or larvte, crawl up the nose, finding their 

 devious way to the sinuses, where, by means of their tentaculce, they at- 

 tach themselves to the mucous membrane lining those cavities. During 

 the ascent of the larvye, the sheep stamps, tosses its head violently, and of- 

 ten dashes away from its companions wildly over the field. The larvae re- 



* The head was cloven with an nx '. It 1.=! proper to eay, however, that various writers speak of having 

 found the gi-ubs in the ethmoid cclU, and indeed in all the nasal cavities. 

 (1038) 



