53 THE HORSE IN MOTION. 



anterior branch of the superficial gluteus, give way, while the weight 

 of the body relieves the extensors of the foot. The function of the 

 limb at this time is to support the weight of the body and prevent it 

 from pitching headlong; and to this end, with the exception of the few 

 small muscles- just mentioned, the entire mass of the muscles of the 

 limb is called into action ; and now that the foot is a fixed point, the 

 semi-tendinosus acts in unison with the others to take the weight of 

 the anterior half of the body. This is the use of all the vast mass of 

 muscular power developed in the haunches and long muscle of the back 

 (ilio spinalis). In this manner there is no act of extension, further 

 than the extension of the body upon the thigh ; it is not until the 

 centre of motion, or head of the thigh, has passed over the foot that 

 extension is possible ; and then the nearer to a horizontal the direction 

 of the force applied, the more effective it will be. When the limb is per- 

 pendicular, the whole force is employed in supporting weight ; but when 

 it is exerted upon the ground at an angle of forty-five degrees, one half 

 of the force is spent in supporting weight, and the other in propulsion ; 

 if it could be exerted horizontally, it is plain it would be exclusively 

 spent in propulsion. From the time when the foot is planted in 

 advance, until the leg has passed the perpendicular, the force is also 

 compound, a part being employed in supporting weight, and the other 

 in resistance which must be drawn from the momentum ; this last is 

 reduced to the minimum by the gradual giving way of the triceps and 

 gastrocnemii, and contraction of the great propellers of the haunch, 

 especially the vastus, which forces the trunk over the supporting limb. 

 The act of propulsion by the vastus begins from the moment that the 

 hind foot takes the ground and its contraction begins. The effect of 

 the contraction of this muscle is to shorten the distance between its 

 two extremities ; one of these extremities is attached to the lower 

 end of the femur and the other to the spines of the sacrum behind 

 the croup, but the course of the muscle is not direct (see Plate V.), 

 being deflected at the head of the femur, and most so when the 

 foot first reaches the ground. At that time it presses with most 

 force against the articulation pressing it forward, so that it extends 

 the trunk upon the limb and forces it forward in the same act. 



