THE HORSE IN MOTION. 



63 



The centre of motion in the anterior extremity may, in its 

 mechanical function, be considered as a joint, and the only kind of 

 joint possible in that position; were it constructed like the corre- 

 sponding joint in the posterior extremity, it would be inevitably 

 broken by the contact with the ground, thrown out as it is in 

 advance of the centre of gravity. For the same reason it is not 

 provided with a collar-bone, or clavicle, as in man and the anthro- 

 poid animals, in whom that bone fixes the shoulder and makes it 

 the centre of motion for the limb. 



On reference to Plate IV., s, the great serratus will be seen as a 

 fan-shaped muscle which has its lower attachments spread out over the 

 first eight ribs. From the attachment to the different ribs its lower 

 border is like a saw, from which its name, "serratus." The artist 

 has vainly attempted to represent the nacreous color, in which it \ies 

 with the mother-of-pearl. This is the tendinous covering to the 

 muscle, and it is much intermingled with tendinous fibres, which 

 limit elongation and take the strain from the muscular fibres when 

 their contraction is not called for. The upper attachment of this 

 muscle is on the inner face of the scapula, or shoulder-blade (Plate 

 XI., a), below the cartilaginous border, with the dark line mark- 

 ing the boundary between it and other muscles. In the centre 

 are seen the gray fibres of tendon, which are continuous below, 

 and enable the animal to rest the muscular fibres and limit their 

 elongation. The space covered on the inner face of the scapula is 

 nine inches in its greatest measurement by two in its least. 



The space below the section of the serratus, as seen in s, s, 

 Plate XL, and between that muscle and those of the inner face of the 

 shoulder-blade, is lined with loose cellular tissue, which, while it con- 

 nects the opposing surfaces, allows of unrestricted motion upon the 

 centre, a, and prevents friction. The body in a standing position 

 rests the weight of the anterior half upon these serratus muscles as 

 upon a sling to which the anterior extremities correspond to crutches. 



But when the foot of one of these limbs is off the ground the 

 serratus is relaxed, and the limb would drop but for another set of 

 muscles, which, though feeble, are sufficient for the purpose which 



