1 6 E VOL UTION AND DISEASE. 



excessive use. Such gradual enlargement of a digit and 

 its hereditary transmission may be demonstrated in 

 Equidce. The modern horse walks upon the greatly 

 enlarged third digit of the hand and foot respectively, 

 the hoof representing the nail. Hidden in the tissues on 

 each side of this functional toe we find vestiges of the 

 second and fourth ; these are familiar to veterinarians as 

 splint bones. 



The researches of palaeontologists have furnished an 

 excellent array of evidence in support of the opinion 

 that the horse has descended from ancestors which 

 possessed five functional digits ; the first and fifth 

 gradually disappeared, the second and fourth still persist 

 but are functionless, whilst the middle one has from 

 increased use attained an extraordinary size. 



In the drawing (fig. 9), a longitudinal section through 

 this large digit of the horse is represented beside the 

 corresponding finger of a man similarly bisected. The 

 homologous parts are indicated by the same letters. In 

 the horse's finger a ' bone is shown, in section, at the 

 junction of the second and third phalanges ; this is called 

 the navicular bone or small sesamoid. Such bones 

 are frequently found in the tendons of man, especially 

 where they glide over bony prominences ; usually they 

 are small, rarely exceeding a split pea in size. In the 

 horse such bones are large and important ; not in- 

 frequently, when the foot is brought violently in contact 

 with hard ground, the navicular bone in one or both feet 

 is broken by the concussion ; the result is permanent 

 lameness, a fractured navicular bone rarely, if ever, unites 

 by bone. In this respect it resembles the great sesamoid 



