68 THE HORSE IN MOTION. 



fifteen dorsal and the lumbar vertebra. Though spread over so much 

 space, the muscular tissue is not correspondingly extensive. The pur- 

 pose was to gain advantage of position as far back as possible to give 

 the most direct action in the line of motion to be produced. As the 

 fibres of its thin tendon (Plate III., /,/") converge forward and down- 

 ward, they become more muscular, and most so just behind the scapula, 

 which is covered at its posterior angle by it, and it is covered in turn 

 in the same region by the dorsal division of the trapezius, g ; after pass- 

 ing beneath the muscles of the shoulder (as seen in Plate XL, d,) its 

 fibres again change to a thin, flat tendon, which unites with the tendon 

 of the muscle, f, and is inserted with it into the internal tubercle of the 

 humerus, about one third of the way from the shoulder to the elbow. 

 If this muscle acted when the foot is off the ground, it is plain that it 

 would flex the shoulder; but its function as a propeller is called into 

 play when the foot is the fixed point, and the limb is supporting the 

 weight of the body, and its articulations are all set. Under such con- 

 ditions it forces the body forward over the foot; but its power as a pro- 

 peller is second to that of the great pectoral (Plate III.,/, p). The 

 limits of this muscle are a little in doubt. It is represented in the 

 plate with the boundaries as given by Chauveau, but it is confounded 

 so closely with the superficial muscle of the skin (paniculus carnosus) 

 on its upper border that it is difficult to separate them. For our 

 purpose, it is sufficiently shown in the plate, extending from the tenth 

 rib over the thorax, covering the serratus magnus as high as the lower 

 border of the great dorsal, and as low as the middle of the thorax, 

 where it unites with its fellow of the opposite side. Its fibres con- 

 verge as they are directed forward, and form a mass of muscle between 

 the arm and thorax so great as to be second in power to no other loco- 

 motive muscle in the body. Its insertion is into the inner tubercle 

 of the head of the humerus, as seen at p, Plate XI., as near to the 

 shoulder joint as possible. The great dorsal may perform two func- 

 tions, flexion or propulsion, as mentioned. The muscle now under 

 consideration has but one. Acting directly upon the angle of the 

 shoulder, there is no loss of its immense power by indirect force, and 

 from the moment that the foot touches the ground, its power is felt in 



