1894.] 



NATURAL SCIENCES OF PHILADELPHIA. 



177 



extensor of the thigh. The great extensor cruris, however, pulling 

 on the kuee-cap and straightening the knee joint, continually moves 

 forward the origin of the gastrocnemius muscle, and the latter pulling 

 on the calcaneum and contracting itself at the same time, draws for- 

 ward the calcaneum faster than the origin moves forward and acts 

 during the whole step. The muscular fibres of the gastrocnemius 

 are so short in the horse, that if the origin were not moved forward, 

 this muscle would reach its limit of contraction long before the end 

 of the step. Thus then, the gastrocnemius is auxiliary to the triceps, 

 not antagonistic. 



To prove this completely, let us cut away in the dissected horse 

 the triceps and every other muscle except the gastrocnemius, which 

 we will contract. Its action is precisely as before. It draws forward 

 the calcaneum, but it extends, not fiexes the knee. 



The paradox disappears when we study a tendon running up the 

 other side of the tibia, the tendinous portion of the flexor metatarsi. 



This tendon takes a greater leverage (fig. 5) 

 on the upper or knee joint that it passes, than 

 on the lower or ankle joint, that it also passes; 

 but the muscle has the reverse leverage. It 

 takes a shorter leverage at the knee than at 

 the ankle. In contracting, therefore, it raises 

 the calcaneum, drawing down the flexor me- 

 tatarsi tendon as shown by the direction of 

 the arrow in the drawing, and the flexor 

 metatarsi extends the knee-joint. Under- 

 standing then the differential action of the 

 gastrocnemius muscle, we might look upon the 

 triceps as the auxiliary of the gastrocnemius 

 in extending the knee-joint. 



The great muscles of the posterior aspect 

 of the thigh, the long vast of the veterinarians 

 (part of glut, max.), the biceps, the semi-tendinosus, the semi-mem- 

 branosus, the gracilis, are inserted not above the knee but below it; 

 not to flex the knee in progression, but to draw on an insertion that 

 in a ditferential manner is moving away from the origin of the mus- 

 cles, in order that the whole of the contractions may be utilized in 

 the whole stride. In progression, then, the croup muscles are auxili- 



FiG. 5. 



13 



