46 THE ARMY HOKSE IN ACCIDENT AND DISEASE. 



Nerves of the foot. 



The nerves of the foot supply feeling or the sense of touch to the 

 parts. The large nerve cord on either side of the limb divides at the 

 fetlock joint into three branches, called the digital nerves — the 

 anterior, the posterior, and the middle. 



The anterior digital nerve passes downward and forward and sup- 

 plies the anterior or front part of the foot. 



The posterior digital nerve, the largest of the three, passes down 

 behind the digital artery to supply the structures in the posterior part 

 of the foot. It gives off a branch which passes through the notch in 

 the wing of the os pedis (in company with the preplantar ungual 

 artery) to supply some of the sensitive laminse; it also sends branches 

 into the os pedis with the plantar ungual artery. 



The middle branch is veiy small, is said to always join the anterior 

 branch, and supplies the sensitive sole and fetlock pad. 



The functions of most of the parts of the foot have been mentioned 

 in passing, but there are some points in connection with the physi- 

 ology of the foot which need to be explained more in detail. 



Expansion and contraction. 



When weight comes upon the leg, the Os pedis descends slightly 

 and causes the sole to descend and flatten. The plantar cushion 

 and horny frog are compressed between the ground below and the 

 structures above; this compression causes them to spread out side- 

 wise, carrying outward the lateral cartilages and bars and the wall 

 at the quarters. This is called expansion. When weight is removed 

 from the leg, the plantar cushion becomes thicker and narrower, and 

 the lateral cartilages and quarters move inward to where they were 

 before expanding. This is" called contraction. The elastic lateral 

 cartilage is merely a flexible extension of the wing of the os pedis and 

 would appear to have been specially designed for expansion and con- 

 traction at the quarters. It is also to be noted that the bars are a 

 provision for this same purpose, since expansion and corTtraction 

 could not take place if the wall formed a solid unbroken ring around 

 the hoof. 



In addition to breaking the j ar when the foot comes to the ground, 

 the plantar cushion has another important use. It assists in the 

 circulation of the blood through the veins of the foot. When weight 

 is placed upon the foot the pressure on the plantar cushion forces the 

 blood upward through the veins; then, when the foot is lifted and 

 the pressure is removed from the homy frog and plantar cushion, 

 the veins of the frog again fill with blood, and this pumping action is. 

 repeated with each step. Proof of this statement is seen when a 

 digital vein is cut, by accident or in experiment. If the horse is 

 walked, a jet of blood spurts out each time he puts the foot to the 

 ground; but if he is allowed to stand the blood flows in a steady 



