44 THE ARMY HORSE. 



On each side of this surface is a groove running forward 

 and terminating in an opening, called the plantar foramen; an 

 artery and a nerve enter the bone and a vein leaves it through 

 this opening. 



On each side of the os pedis, extending backward, is a pro- 

 longation, called the wing. Each wing is divided hy a notch 

 and then by a groove, which runs forward on the outside of the 

 bone ; an artery lies in the notch and groove. 



The OS naviculaTis is an irregular bone situated behind and 

 below the os coronse and behind the os pedis, articulating with 

 both bones. Its long axis is perpendicular to the axis of the 

 foot. The extremities of the bone are attached to the wings of 

 the OS pedis; the inferior surface is covered with cartilage, 

 which forms a smooth surface for the movements of the tendon 

 of the flexor pedis perforans muscle. 



Elastic structures of the foot. 



All of the parts of the foot, except the bones, are more or 

 less elastic or " springy " and yield when pressure is applied; 

 but certain parts have a very high degree of elasticity, their 

 special use being to overcome the effects of concussion or jar 

 when the foot strikes the ground and to prevent injury, and 

 these parts are referred to as the elastic structures of the foot. 

 They are the lateral cartilages and plantar cushion, or fatty 

 frog, as it is sometimes called. 



The lateral cartilages are thin plates of cartilage, one at- 

 tached to the top of each wing of the os pedis, and extending 

 backward and upward so far that their upper borders may be 

 felt under the skin above the coronet at the heels. 



The plantar cushion is a very elastic wedge-shaped pad, 

 which fills up the space between the two lateral cartilages on 

 the sides, the sensitive frog below, and the flexor pedis per- 

 forans tendon above. 



The point or anterior part of the plantar cushion extends 

 forward to the ridge which separates the sole from the tendin- 

 ous surface of the os pedis. The base is covered by the skin 

 above the heels. 



Sensitive structures of the foot. 



Over the bones and elastic structures of the foot is found a 

 complete covering of very sensitive flesh, and from each part of 



