136 EXAMINATION OF HORSES 
of oil, in which case the parts must necessarily be tense 
from over-distension. 
Lastly, if there is disease of the joint you will have 
lameness, heat, and over-distension; but as a matter 
of course your test lies. in setting the machine in motion 
and seeing what effects are produced, which will be most 
visible, not zmmediately after the motion, but when the 
horse has stood awhile and cooled. 
Thorough-pin is-precisely of the same nature as bog- 
spavin ; only instead of a joint, it is a tendon surrounded 
by a lubricating sheath, that has to act like a pulley, 
and requires well lubricating. This sheath gets over- 
distended with oil (synovia). The perforating flexor of 
the foot has its origin quite externally at the back of 
the tibia, and becomes tendinous above the hock and 
passes over the zvséde of the: hock, and again gains the 
middle line after it has got well below the hock, so that 
it has quite a spiral course, which makes it lie close to 
the internal surface of the os calcis, which is rendered 
concave to receive it and over which it glides. In 
passing thus obliquely from without inwards, it has to 
pass between the point of the os calcis and the back of 
the tibia, and the straighter the hock the nearer does 
the point of the os calc’s approach the back of the tibia, 
therefore the more does this synovial sheath get pressed 
upon in straight hocks. This explains its more frequent 
appearance on good upright hocks, whose ossa calces 
are usually shorter on account of less leverage being 
required, and whose ossa: calces: are necessarily closer to 
