6 NOTES ON SHOEING OF HORSES. 



its toughness, and without it quickly becomes dry, 

 hard, and brittle. 



Over the whole exterior of the crust is laid a 

 gluey superficial layer, by which this oily material 

 is preserved from exudation. So long as this outer 

 layer is maintained uninjured, the crust retains its 

 moisture, toughness, and strength. 



The crust from its structure is well calculated to 

 sustain weight. Its fibres are each of considerable 

 substance ; their strength is greatly augmented by 

 being closely cemented together ; the cement, which 

 binds them together, is protected from evaporation 

 by the external gluey layer ; and lastly, the fibres 

 themselves stand in the position most favourable for 

 sustaining weight and resisting the pressure which 

 falls on them. Any row of props, as is well known 

 to all builders, will sustain greater weight, when 

 placed perpendicularly to the thrust, than when in 

 any other position. The weight of the horse, whether 

 in a state of rest or in motion, falls nearly per- 

 pendicularly on the fibres of the crust, when placed 

 at an angle of about 45° to the ground. 



In the sole, on the other hand, all these conditions 

 favourable for sustaining weight are wanting. The 

 fibres are much less substantial than those of the 

 crust, they are not so closely connected together, 

 and, lastly, they are placed in layers in a horizontal 

 position. The sole, therefore, from its construction, 

 is unable to sustain weight or pressure. 



