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614 A MANUAL OF VETERINARY PHYSIOLOGY 



is turned outwards ; when it is straightened, the stifle is turned 

 inwards, and in some horses the effect of the screw- turning 

 movement is to cause the foot, when it comes to the ground, 

 to be twisted, and the point of the hock to be thrown out- 

 wards. This gives the appearance of a wrench, but is natural 

 to some horses. Animals with a considerable space between 

 the last rib and the stifle-joint do not need to have their stifles 

 turned outwards. With the horse the last rib and stifle are so 

 close together that the latter has to be directed away from the 

 abdominal wall. 



The movement between the small bones forming the hock-joint 

 is very slight, and is not of a to-and-fro sliding motion, such as 

 might be expected, but partakes of the nature of a rotation 

 or twist on each other. This fact will be understood when it is 

 remembered that the centres of the small bones of the hock are 

 attached to each other, and to the astragalus and metatarsus, 

 by interosseous ligaments centrally placed, which practically 

 prohibit any movement but that of slight oblique twisting. 

 The fact is also revealed by the friction marks in diseased 

 joints. 



The range of motion between the tibia and astragalus is con- 

 siderable, yet it is not fully exercised in all paces. It is only 

 in jumping and the gallop that the astragalus moves from end 

 to end on the tibia. In order to prevent excessive flexion, 

 two stops or buffers exist on the tibia, which come into contact 

 with two rests on the astragalus. The outer stop is the larger 

 of the two, but owing to the oblique face of the tibia, the inner 

 one is the first to make contact. When the hock- joint is fully 

 flexed, only the anterior part of the tibia is in contact with the 

 astragalus ; the posterior part is raised from the trochlea, and 

 a space exists between them. The line of pressure through the 

 hock falls mainly on its anterior part, and on the inner rather 

 than the outer side. This is the region most frequently affected 

 with disease. The tissues at the back of the hock suffer when 

 the fore-hand is raised during jumping, and may also suffer 

 strain when the hind-limbs receive the weight of the body in 

 the gallop ; but the chief trouble affecting the hock is that 

 occurring to the bony structures. 



The Functions of the Limbs in Relation to the Causes of Lameness. 

 — The bones forming the flexible columns of the limbs, with their 

 tendons, ligaments, and enveloping tissues, are exposed to strain. 

 It is evident that the strain is greatest during movement, but it 

 requires some little experience with horses to realise the stress 

 the parts are exposed to even when the animal is doing no work. 

 For instance, laminitis is common when horses are standing in a 

 fixed position, as on board a ship, though nothing more than the 

 weight of the body is being supported. A horse injured in one leg 



