MUSCLE 233 



3rd Class. — Power between fulcrum and weight. In 

 raising a weight placed on the dorsal aspect of the toes by 

 the contraction of the extensors of the foot, we have the 

 weight at the toes, the power at tlie tarsus, and the fulcrum 

 at the ankle. 



In the joints of animals actions involving tlie principle of 

 each of these levers may be found. 



5. Special Mechanisms in the Horse. 



(1) The Limbs of the Horse- 



The condition of the feet and legs is the chief limiting 

 factors in work production in the horse. An intimate 

 knowledge of the anatomy of the various structures involved 

 must be obtained in the dissecting room. 



The Foot. — The weight of the horse is transmitted 

 through the second phalanx to the foot (fig. 116). 



The articular surface of the second phalanx is larger than 

 that of the third, so that its posterior part rests upon the 

 sesamoid bone which is supported by the tendons of the flexor 

 muscles and further held in position by the sesamoid ligaments. 

 This jaelding articulation assists in reducing and distributing 

 shock. The tendon of the flexor muscle rests upon the 

 digital torus (the plantar cushion). This is a pyramidal 

 shaped mass of yellow elastic and white fibrous tissue which 

 stretches between the cartilages of the third phalanx to 

 which its edges are attached (fig. 117). The superficial 

 (volar) surface of this structure is arched, but becomes 

 flattened out under pressure. The cartilages to which it is 

 attached are elastic and yield under pressure, being pushed 

 out slightly. The digital torus rests upon the elastic cuneus 

 (the frog), which in the normal unshod hoof is on a level 

 with the wearing edge of the wall. The sole of the foot 

 (fig. 118) is concave. When the weight of the animal is 

 put on it, the arch tends to flatten out, and at the same 

 time there is slight expansion of the heel. This is provided 

 for by the walls at the heel being deflected inward to form 

 the bars instead of being continuous to complete the circle. 



