192 THE STEUCTUEE OF THE HOESE. 



hoofs, are met with throughout the Mammalian class, 

 with numerous varieties or modifications of each, 

 and transitional 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 re- 

 spect 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 transversely 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 channeled 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 dis- 

 tal phalanges is related to the great development of 

 these bones, and to increasing the mechanical advan- 

 tage of the flexor tendon which passes over it. Al- 

 though not present in most mammals, it is not 

 peculiar to the horse, being found, though on a 



