THE HORSE IN MOTION. 



47 



where the angles are right angles, and the application of power is direct, 

 there is less need of composition of forces ; but the design of nature 

 was higher: beauty was superadded to power, and for this end great 

 sacrifices of power were made. Though difficult of demonstration, it 

 may be taken for granted that at full speed the adduction and abduc- 

 tion of all the muscles in action counterbalance each other; if they did 

 not, either the feet would interfere or they could not be brought to 

 support the centre of gravity, and in either case the animal might fall. 



How is it possible for the student to learn the action of the machine 

 when the muscular forces are represented as chiefly composed of 

 adductors and abductors, as if the animal was designed to move side- 

 wise like a crab.'' These names maybe perfectly proper to express 

 the action of their analogues in man ; for man, of all his relations, has 

 the most inefficient locomotive apparatus, but the greatest diversity of 

 action in his extremities. 



Leaving the muscles of the haunch, we descend to those of the leg. 



The t7nccps femoris (Plates III., IV., V., IX., /, /) is the great 

 muscle that occupies the front of the thigh. As its name implies, it 

 has three heads. The middle one has its upper insertion in the 

 smooth facet of the pubis, directly above the acetabulum, or cup, in 

 which the head of the femur rests (Plate II., 6), and is called the 

 rectus. The other two heads are attached to the broad face of the 

 femur, as close as possible to the head of the bone without inter- 

 fering with its free action. Its length is eleven inches only, but 

 its circumference is twenty, and its weight nine pounds. It cannot 

 be separated into distinct muscles, and it acts as a unit in extend- 

 ing the leg forward through the patella, or knee cap, its point of 

 insertion ; but the rectus, or middle head, being attached to the pel- 

 vis, has the power of moving the femur on which it lies, as well as of 

 extending the leg in common with its fellows, so that the action is to 

 extend both bones on a line forward ; but the patella is not, like that in 

 man, a part of the knee, or articulation, between the femur and the tibia; 

 it has a place of its own. The front portion of the lower extremity of 

 the femur is elevated or built up, and furnished with a trochlea, or 

 grooved surface, with cartilage and synovial membrane, expressly for 



