682 A MANUAL OF VETERINARY PHYSIOLOGY 



lifting up one fore-foot, and so causing the horse to throw 

 double weight on the other limb, an expansion of -^ of an inch 

 for one half of the foot, or -fa of an inch total increase in width. 

 Naturally, during locomotion a greater expansion than this occurs. 

 The question may be asked, What advantage can be gained 

 by such a small increase in the width of the foot ? Small as the 

 increase is, it still makes all the difference between a yielding 

 and an unyielding block of horn being brought to the ground ; 

 it ' gives ' instead of offering resistance, and this ' give ' is 

 sufficient to prevent the hoof from being fractured, while the 

 pad which has largely caused the expansion has acted as a 

 buffer and assisted to destroy concussion. 



There is no point in the physiology of the foot which has given 

 rise to greater diversity of opinion than the question of ' expan- 

 sion ' ; modern investigations completely support the views of 

 the earliest observers. The retraction of the coronary edge 

 of the foot in front and the sinking behind are accompanied by 



a tense condition of the parts 

 which, since the days of Bracy 

 Clark, have been regarded in the 

 light of an elastic ring or support 

 to the pedal joint. The tense 

 condition is due to the change in 

 the shape of the coronary edge, 

 Fl ?- 24 ?-~ B ™TrZZZJ^: but whether this is capable of 



CERTAINING THE COMPRESSION . \ 



of the Wall. affording support is not evident. 



In addition to the changes in 

 the coronary edge of the foot during the period of expansion, 

 another condition is present — i.e., compression of the wall 

 under the influence of the body weight, which produces a 

 diminution in its height. This can be roughly demonstrated 

 in the 'following manner : If a portion of the wall at the 

 heel be cut away so as just to clear the shoe when the 

 latter is fitted, it will be found on placing weight on the 

 limb, by lifting up the opposite fore-foot, that the wall has 

 descended sufficiently to touch the shoe. The experiment may 

 be rendered less free from objection by removing a piece of the 

 wall, as in Fig. 243. If this is made sufficiently large to admit 

 a penny when the foot is off the ground, the coin cannot be 

 introduced into the slot when the weight is placed on the foot. 

 The only explanation which can be afforded is that given above— 

 i.e., the wall has undergone sufficient compression to allow the part 

 which was originally clear of the shoe to come in contact with 

 it, and to produce this it must have diminished in height. 



The Descent of the Pedal Bone is the last factor employed in 

 saving concussion. The existence of this has been as strenuously 



