16 EVOLUTION AND DISEASE. 
excessive use. Such gradual enlargement of a digit and 
its hereditary transmission may be demonstrated in 
Equide. 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 paleontologists 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 
eradually 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 
