734 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



muscular action. When the body is uniformly supported on all four 

 extremities, in the fore limbs the line of direction passes from the shoulder 

 through the centre of the elbow-joint in the axis of the forearm, through 

 the centre of the knee-joint in the axis of the ulna, through the centre 

 of the pastern-joint perpendicular to the ground behind the ball of the 

 foot. Three principal angles are thus formed whose degree depends 

 partly on the angle between the scapula and vertebrae and partly on the 

 relative lengths of the different bones. 



In the case of the anterior extremity the line of direction of the 

 lower bones is almost vertical, but in the case of the upper bones becomes 

 considerably inclined; thus, the scapulo-humeral angle tends constantly 

 to become smaller and smaller on account of the depression of the 

 superior extremity of the scapula and by the projection anteriorly of 

 the scapulo-humeral articulation. This depression and projection are 

 hindered by the action of numerous muscles. The most important 

 muscle concerned in the fixation of the upper extremity is the serratus 

 magnus, which, arising in numerous fan-shaped bundles from the five 

 posterior cervical vertebrae and the first eight ribs, converges to be 

 inserted in the ventral surface of the scapula. This muscle, with its 

 fellow, serves as a muscular sling in which the body is supported 

 between the fore limbs, the axis of motion of the shoulder-blade pass- 

 ing through its insertion in the scapula. 



The superior extremity of the scapula is sustained by the rhomboid 

 muscles, which draw it upward, as well as by the trapezius, which tend to 

 elevate and advance this bone, so opposing the depression of the scapula 

 through the weight of the trunk. These muscles thus give fixity to the 

 scapula. Further, the anterior projection of the scapulo-humeral angle 

 is prevented by the greater and lesser pectoral muscles, which by their 

 contraction tend to retract this angle. The obliquity of the humerus 

 tends to become exaggerated during standing as well as at each act of 

 striking the foot upon the ground. The pectoralis major and the infra- 

 spinalis muscles, as well as the coraco-radii, tend to prevent this, the latter 

 muscle being especially efficacious, acting not only as a muscle, but a 

 band of unyielding tendinous material running through it enables it to 

 act as a ligament preventing exaggerated flexion of the humerus on the 

 shoulder. That the coraco radii should fulfill this function it is neces- 

 sary that its insertion in the inferior extremit}' should be fixed. This 

 fixity is accomplished by the five olecranon muscles. At the elbow-joint 

 the lower end of the humerus is fixed by the strong lateral ligaments; 

 reduction of the elbow angle being prevented by the fixation of the 

 olecranon through the contraction of the extensors of the forearm, 

 and anterior deviation by the tendinous expansion of the long flexor of 

 the forearm, its tension increasing with the weight on the shoulder. 



