320 CHAPTER 41. 



639. Causes of Bursal Enla'-gements. 



When any cause, such as over-exertion, produces irritation iu the part, 

 the Synovial membrane is excited by the irritation to throw out an 

 increased secretion of oil. This increased supply must not be regarded 

 as an evil in itself. On the contrary it is useful in lessening the irrita- 

 tion, and is, in fact, a bountiful provision of nature for that purpose. 

 Similarly, when a sprain occurs in the ligaments of a joint or in a tendon, 

 an increased secretion of synovia is poured forth with the same object. 



The liability to the occurrence of such causes is, of course, greatly de- 

 pendent on the conformation of the animal. Upright shoulders, fetlocks 

 or hocks, and all other points of conformation which do not give due 

 elasticity to the frame in action, are liable to cause Bursal Enlargements. 



Chronic inflammation of the joints, which is often found as a result 

 of pneumonia, influenza, and sometimes of general debility, is another 

 common cause. 



Though overwork, sprain, faulty conformation, or chronic inflammation 

 of the joints may be set down as the usual causes of Bursal Enlargements, 

 yet they sometimes occur without any such violent exciting causes, and 

 can then only be attributed either to a special irritability of the Synovial 

 membrane, on account of which it is excited to increased action on very 

 slight provocation, or to weakness of the coats of the blood-vessels of the 

 membrane, through which an undue effusion takes place. 



From these general remarks on the nature of Synovial membranes, 

 and on the causes of Bursal Enlargements, we pass to the consideration 

 of the particular affections, which bear various names accoi'ding to the 

 part in which they appear. 



640. Bog Spavin. 



Bog Spavin is distension of the bursa of the true hock joint. This 

 joint, as explained in the Chapter on Spavin, No. 38, consists only in the 

 articulation of the tibia and astragalus. The other bones of the hock do 

 not enter into it. The swelHng shows itself primarily in front, because 

 in that part the capsule is large and loose, and not bound down by bones 

 or ligaments, and therefore it is easily distended. 



Bog Spavin is most frequently found in upright shaped hocks, because 

 that formation induces concussion and irritation. It is also commonly 

 found in weak hocks of any description, because in them any over-exer- 

 tion is likely to be injuriously felt and therefore is very liable to cause 

 irritation. Such hocks are also more liable to sprain. 



Of the treatment of Bog Spavin we shall speak hereafter in common 

 with that of other Bursal Enlargements. 



** 641. Tlwrougli-pins. 



Thorough-pin is the name given to a Bursal enlargement, which occurs 

 at the upper and back part of the hock beneath the great extensor pedis 

 tendon. The swelling appears sometimes on one side only, but more 

 frequently on both sides. 



