THE FOOT 679 



admit of increase in width occurring at the posterior part of the 

 foot without destroying the union of the two sets of leaves. 

 Further, by their connection with the vascular system of the foot, 

 their elastic movements materially assist the circulation. The 

 primary use of the lateral cartilages is to render the internal foot 

 elastic, and admit of its change in shape which occurs under 

 the influence of the weight of the body. The alteration in the 

 shape of the foot is brought about by pressure on the pad, which 

 widens and in consequence presses on the bars. The pressure 

 received by the pad is also transmitted to the plantar cushion, 

 which likewise flattens and spreads under pressure. Both of 

 these factors force the cartilages slightly outwards. When the 

 posterior wall recoils the cartilages are carried back to their 

 original position. Should this elastic cartilage under pathological 

 conditions become converted into bone, its functions are 

 destroyed, and lameness may occur. It can be demonstrated 

 that by surgical interference the hoof can be made permanently 

 wider, and thereby rendered capable of accommodating lateral 

 cartilages which have undergone an increase in size as the result 

 of ossification.* 



Anti-Concussion Mechanism. — The special physiology of the 

 foot is a consideration of the factors whereby the parts are saved 

 from concussion, in spite of wear and tear, batter and jar. The 

 weight carried on each fore-foot when the horse is standing is 

 rather more than one quarter the weight of the body ; during 

 locomotion the amount varies from half the weight in the trot 

 to the entire weight in certain stages of the canter and gallop. 

 The mechanisms which exist in the foot to save concussion are 

 not only intended for the protection of the foot, but also to save 

 the limb, and they may be tabulated as follows : 



1. The yielding articulation in the pedal joint. 



2. The increase in the width of the foot when the heels come 

 to the ground, known as expansion. 



3. The elastic foot-pad. 



4. The slight descent of the pedal bone, and with it the sole. 

 Expansion is a term warranted by custom, though perhaps not 



free from objection. It indicates the fact that the foot opposite 

 to the heels becomes wider when the weight ■ comes on them 

 (Fig. 240). The increase in the width of the foot is due to a 

 temporary alteration in the shape of certain of its structures. 

 As a matter of fact, an increase in the width is not the 

 only change which occurs ; it can be shown that the heels at the 

 coronary edge sink closer to the ground, while the coronary edge 

 of the wall in line with the toe of the foot retracts, or travels 



* ' Operation for the Cure of Lameness arising from Side-Bone,' Veterinary 

 Journal, vol, xxv. 1887. 



