APPENDIX TO SECOND EDITION. 167 



a well-formed foot, the base of the frog ought to oc- 

 cupy a sixth part of the circumference of the circle 

 of the hoof ; in the center of the frog is a horny con- 

 ical cavity of considerable depth, which protects the 

 partially cleft foot from further rupture, adds to the 

 elasticity J secicres a firmer hold on loose soils, and pass- 

 ing above into the substance of the sensitive frog 

 serves to unite firmly the two halves of the foot, 

 which are completely divided in ruminants ; this 

 horny cone has been called the frogstay or bolt. The 

 sensitive frog falls into the inverted arch of the horny 

 frog, which are thus held mutually in place and pre- 

 served from external shock. The sole is an irregular 

 plate of horn, closing up the lower opening of the 

 foot, of an arched form, abutting everywhere against 

 the sides of the wall, another contrivance for securing 

 elasticity. The foot of the horse, therefore, though 

 solid in front, is partially cleft behind, so that the 

 terms solidimgula and solipoda can not strictly be ap- 

 plied to it ; indeed a solid, continuous, unyielding circle 

 of horn would be very painful if not entirely useless as 

 an instrument of active progression ; this beautiful 

 structure, however, is sadly interfered with in almost 

 all methods of shoeing. Immediately under the hoof 

 are extensive cartilages, attached to the last two 

 bones, protecting the upper part of the structure and 

 adding greatly to the elasticity of^ the foot, and per- 

 mitting the movements of the cofifin bone within the 

 hoof ; in old horses these cartilages may become par- 

 tially ossified, and are then called ring-bones. Under 

 the hoof is also a very sensitive and vascular layer, 



