63 THE HORSE IN MOTION. 



to a common centre on the inner face of the upper border of the 

 shoulder-blade, or scapula, as seen in Plate XL, 5, s. 



When this muscle is recently exposed it presents delicate nacre- 

 ous tints rivalling pearl. The artist has suggested them only in his 

 drawing. This pearly coat of the muscle is tendinous in its struc- 

 ture, and extends over the w^hole exterior surface of the great serra- 

 tus. These tendinous fibres extend throughout the muscle, but are 

 in greater proportion near the centre or long axis (Plate XL, a). 

 These tendinous fibres, concentrated at a, may be considered the 

 centre of motion for the whole limb when supporting the weight of 

 the body, whether acting alone, or in conjunction with one or more 

 of the other limbs, and whatever may be the direction of its axis 

 with reference to the trunk ; but this centre of motion must not be 

 confounded with the centres of motion existing in the joints ; it 

 holds a corresponding position with the " whirlbone," or hip joint 

 of the posterior extremity. This intermixture of muscular and 

 tendinous fibres existing in this muscle is found in others, as the 

 deep gluteus described in the last chapter, enabling it to perform 

 the functions of both muscle and ligament. The tendinous fibres, 

 which are in the greatest proportion in the long axis, when put 

 to their tension absolutely limit elongation to that degree, and are 

 useful when the animal is standing ; as these tissues are incapable 

 of fatigue, so he has no occasion to rest them. With the aid of 

 another muscle, which we shall describe further on, having the same 

 characteristic construction as the serratus, the horse is enabled to 

 stand in his stall all day without resting either of his fore legs ; 

 while in the hind leg the labor falls upon the triceps (Plate IV., ^), 

 of pure muscular fibre, and he will be observed to rest his hind legs 

 alternately. (See page 50.) 



The muscular fibres of the serratus are most abundant at the 

 anterior and posterior borders. The former aid in preventing shock 

 when the foot first takes the ground, and the latter in giving the 

 final propulsive effort when it leaves it ; and by their joint action 

 they relax the tendinous fibres, or bands, which, being passive, have 

 no such power in themselves. 



