180 THE STEUCTUEE OF THE HOESE 



not meeting behind, and with the interval filled by a 

 mass of epidermis of great thickness but somewhat softer 

 structure than the nail proper, which is distinguished as 

 the c sole ' of the hoof. This corresponds in position to 

 the rounded end of the human finger. 



These three principal modifications of the same struc- 

 tures, described respectively as nails, claws, and hoofs, 

 are met with throughout the Mammalian class, with 

 numerous varieties or modifications of each, and tran- 

 sitional conditions by which they pass one into the other. 



The horse shows the most extreme development of 

 size and perfection of structure to which the hoof has 

 attained, even considerably exceeding in this respect 

 his nearest living allies, the asses and zebras. The bone 

 which constitutes its support resembles that of man in 

 the way it is jointed on to the bone above by a trans- 

 versely extended concavity, and in having the ends of the 

 long tendons of an extensor and flexor muscle inserted 

 into it, one on the anterior and one on the posterior 

 surface ; but the bone is wonderfully different in shape, 

 being very short, greatly expanded laterally, and ending 

 below in a sharp but wide, nearly semicircular, distal 

 edge. The bone is remarkable for its dense, almost 

 ivory-like character, and is channelled and perforated to 

 allow the passage of blood-vessels. The presence of a 

 large sesamoid bone (the ' navicular ' of veterinarians) 

 behind the articulation between the middle and distal 



