THE FOOT 683 



denied as the expansion of the" wall, but there is, however, no 

 difficulty in demonstrating it ; the value of such a function is 

 undoubted. Concussion to the sensitive foot is prevented by 

 a slight up-and-down play between the sublaminal tissue and 

 the pedal bone ; as the weight comes on to the foot the pedal 

 bone descends slightly, to rise again when the weight is taken 

 off the limb.* As the pedal bone descends, the sole on which it 

 is resting also descends slightly and comes nearer to the ground ; 

 this is one reason why the sole is concave instead of flat. The 

 soft horn uniting the sole and wall specially provides for the slight 

 descent of the sole. The descent of the internal foot saves 

 concussion, in the same way that it is easier to catch a cricket- 

 ball with a retreating movement of the hand than by rigid 

 opposition ; further, it facilitates the circulation. The descent 

 of the pedal bone is a most important physiological factor, and 

 one of the safeguards of the sensitive foot. 



Vascular Mechanism. — Lying as the foot does farthest from 

 the heart, it is natural to inquire how it is that the blood is able 

 to circulate through it so thoroughly, and whether other means 

 are at hand for assisting the force of the heart in facilitating the 

 circulation. Though the contraction of the left ventricle is 

 sufficient under ordinary circumstances to bring the blood back 

 to the right side of the heart, it is doubtful whether it would 

 be wholly sufficient to empty the foot of blood and keep the 

 considerable plexus of veins full. This plexus is shown in Fig. 226 , 

 p. 662, which is a reproduction from a photograph of a corrosion 

 injection. f The figure conveys very accurately an idea of the 

 remarkable venous arrangement of the foot. The venous cir- 

 culation is assisted by two movements in the foot — viz., the 

 expansion and recoil of the outer foot, and the descent and eleva- 

 tion of the inner foot. There is no difficulty in seeing the move- 

 ment imparted to a column of fluid circulating in these parts, 

 for if a plantar vein be divided and the horse made to walk, 

 every time the foot comes to the ground the blood spurts out 

 from the vein as if from an artery ; when the foot is taken off 

 the ground the stream of blood becomes greatly reduced. A 

 perfect pumping action is in this way produced. The mechanism 

 can also be demonstrated on the dead limb, by placing a mano- 

 meter tube filled with water in each plantar vein, and then pressing 

 downward on the limb, thus roughly imitating the weight on 



* Seventy years ago W. C. Spooner wrote : ' On trying the elasticity of 

 the membrane which connects the laminae to the bone, I found, somewhat 

 to my surprise, that it yielded considerably when pulled downwards and 

 very slightly when pulled upwards ' {op. cit.). 



f The figure appeared in an article by Dr. C. Storch, of Vienna, on 

 ' The Venous System of the Horse's Foot,' Oesterreichischen Monatschrift 

 fur Thievheilkunde, 1893. 



