THE POSTERIOR EXTREMITIES. 85 



three intentions : First, it gives lightness as well as solidity 

 to the foot. Secondly, it affords protection to the nerves 

 and vessels which ramify within its interior ; and which could 

 not travel on the outer surface without suffering serious 

 pressure from the great weight of the horse. Thirdly, it 

 gives ample hold to the various structures covering its 

 exterior ; and thus bestows strength and durability to the 

 entire machine. 



The nut, the shuttle, the quiltor, or the navicular bone, is 

 situated behind the coffin bone, between the two wings. Its 

 upper surface is continuous with the articulatory surface of 

 the coffin bone. Its lower surface rests on the perforans 

 flexor tendon ; which tendon also rests upon the insensible 

 frog ; which again has the sensible frog and the tough and 

 flexible horn of the external fi'og on its lower side. Its 

 motion is downward ; and in this direction it would appear 

 to be amply protected. By its upper surface it helps to 

 sustain the vast weight of the animal ; yet between it and 

 the coronet bone there is only synovia to break the force. 

 Nor does it require any thing more ; however much to the 

 contrary it may appear to the inexperienced reader ; for to 

 the force from above it is yielding. The upper surface of 

 the bone is never injured ; but the lower surface driven 

 further downward whenever the coronet bone travels back- 

 ward, — the inferior surface though, as it would seem, amply 

 protected by the synovial capsule ; by the perforans tendon ; 

 by the sensible frog ; by the insensible frog ; and by the 

 tough horny frog, — is the situation of one of the most 

 terrible and fatal scourges that horse-flesh is heir to, viz. 

 navicular disease ; because, in this direction, the bone is 

 propelled against and bruised by any inequality of the 

 ground. 



THE POSTERIOR EXTREMITIES. 



These differ much from the anterior ; not only in the 

 strength of the parts generally, but in the length and direc- 

 tion of the bones entering their formation. 



The femur, or thigh hone {Skel. F), is the largest, thickest, 

 and strongest in the body. It is, however, short in the 

 horse, as compared with that of most other animals : its 

 surface exhibits eminences, and indentations for the attach- 



