THE FOOT. 61 



The navicular bone is also a sesamoid bone ; i. e., it is 

 formed in tendon. It is a short bone in structure, but rather 

 long and slender in shape, and placed crosswise just back of the 

 articulation between the second and third phalanges. Its in- 

 ferior surface is smooth except for a slight ridge in the center. 

 The tendon of the deep flexor muscle passes over this surface 

 on its way to its insertion on the bottom of the third phalanx. 

 The ends of the navicular bone unite on each side with the wings 

 of the third phalanx by tough fibrous tissue. 



The horny hoof. — This is to be regarded as an appendage 

 of the skin. It covers all of the third phalanx and part of the 

 second. It is divided for study into wall, sole, and frog. 



The wall is that part which may be seen when looking at 

 the hoof in front and at the sides. It is composed of horny, 

 tubular fibers, which grow from the coronary band. Between 

 these tubular fibers is a material which holds them together. 

 The wall is divided into toe, quarters, and heels. At the top is 

 a groove into which the coronary band fits, the crown of the 

 hoof. The wall unites below with the sole. The smooth shin- 

 ing layer which should cover the whole surface of the wall is 

 periople. The duty of this layer is to prevent undue evaporation 

 from the hoof. Beneath the periople is the horny wall which 

 gives shape to the foot and protects the soft parts within. Inside 

 this hard layer come the laminae, 500 to 500 thin leaves of horny 

 material which dovetail in with the sensitive laminae. 



The sole is composed of horny material resembling that of 

 the wall. It is concave below, convex above. Its outer border 

 unites with the wall at the "white line." Its upper surface is cov- 

 ered by the vascular velvety tissue. 



The frog is a "V" shaped elastic pad with a depression, 

 called by horsemen "the cleft," in its ground surface. It is lo- 

 cated between the bars on each side, and below the plantar cushion. 

 Its function is to act as a pad and to lessen jar when traveling. 



The matrix (horn generating membrane). — This fits inside 

 the hornv part of the hoof and covers like a stocking the other 

 parts within the hoof. It is from this that all the horny parts 

 grow. It is composed of three parts: (1) coronary band; (2) 

 sensitive laminae ; ( 3 ) velvety tissue. 



The coronary band is the elastic ring that may be felt at 



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