262 



THE HORSE IN SERVICE 



borne down upon, and in the normal unshod foot further de- 

 pression is opposed by the contract of the frog with the ground. 

 The structures compressed between the pedal or coffin bone 

 above and the unyielding ground or roadway beneath are elastic, 

 and yield in the line of least resistance, which is laterally. 



Fig. 134a. Longitudinal median section of the foot, showing the internal structure. 



Three bones form its osseous base and permit it to accomplisli its various movements. 

 These are: the third phalanx or pedal bone (a); the second phalanx, or coronanj bone {b)\ 

 finally, the navicular, or fniall sesamoid bone (r), situated behind the preceding and comple- 

 menting the articulation which the other two form. 



Short, strong ligaments consolidate the joint on the sides, while two wide fibro-cartilagi- 

 nous plates, lateral cartilages of the third phalanx, intimately united to this bone, seem like 

 two elastic and diverging springs, placed on the outside and on the inside of this bone, to 

 prevent it from descending or rocking too suddenly in the hoof at the moment when the 

 latter strikes the ground. 



Two strong, expanded tendons terminate upon the third phalanx: the anterior (d) carries 

 it into extension; the posterior (e) permits, on the contrary, the flexion of this bone upon 

 the OS coronae. It glides over the inferior face of the navicular bone by means of a synovial 

 sheath designated under the name small sesamoid sheath (better called the naiictdar sheath). 



Finally, a voluminous fibro-elastic cushion (?/), called the plantar cushion, bifurcated 

 behind and pointed in front, is placed under the flexor tendon, to which it serves as a flexible 

 bufl'er when the foot has reached the ground. All pressure upon the hoof from below up- 

 ward tends to depress it and force it against the lateral parts, where it is maintained by the 

 two elastic cartilages indicated above. (Gonbaux and Barrier.) 



This sidewise expansion of the internal structures of the foot 

 presses the bars and lateral cartilages outward, and with them 

 the wall at the quarter, thus increasing the transverse diameter 

 of the foot from one-fiftieth to one-twelfth of an inch. As this 

 expansion is most marked in the back half of the foot, there is 

 a corresiDondino^ narrowing of the hoof head in front. Fig. 134a. 

 Interference, by shoeing or otherwise, with this lateral ex- 



