44 THE HORSE IN MOTION. 



cession of bounds, it is developed enormously in comparison with 

 the gluteus. 



The semi-tctidinosus is represented in Plates III., V., IX., s, s, s, 

 where its situation is shown immediately behind the vastus. It 

 has two origins, one from the sacral spines and the first of the tail 

 bones or their ligaments, the other from the lower face of the 

 ischium (Plate V., i), below which they unite. It divides into three 

 branches ; the central is attached to the strong fascia covering the 

 muscles of the calf, the other two reach forward to be attached to 

 the same common fascia, one on the inner and the other on the 

 outer face of the leg; the latter is spread out as far forward as the 

 insertion of the long vastus ; the inner to a corresponding position 

 on the inner- face. These lateral branches overlay the muscles of 

 the calf, or gastrocnemii, and give that compressed form that distin- 

 guishes the calf of the horse's leg. It is a powerful muscle. Its 

 weight is eleven pounds. The distance of its origin at the spine 

 to its insertion at the head of the tibia is twenty-eight inches. 

 The part of the muscle which has its origin at the ischium, to the 

 same point of insertion, is nineteen inches, and its greatest circum- 

 ference is ten inches. 



The action of this muscle cannot be represented by any abstract 

 terms. It has two functions : it lifts the leg when the act of 

 propulsion is complete, flexing the leg upon the thigh until the 

 line perpendicular from the centre of motion is passed, when it 

 relaxes, while the extensor proper of the leg, the triceps femoris (/, /), 

 carries the foot to a new position in advance. As soon as the foot 

 is upon the ground and the limb feels the weight thrown upon it, 

 then the full power of this muscle is called into play, no longer as 

 a flexor, and not as an extensor, nor even as a propeller, but as a 

 supporter, which character it performs until the direction of its fibres 

 passes the perpendicular, when they cease to act until the next stride 

 begins, so that when the foot is off the ground in the first quarter 

 of the stride it is a flexor; it is inactive in the second quarter, 

 and a supporter in the third, while it plays no part in the fourth. 

 The importance of the proper understanding of the action of this, 



