THE FOOT 655 



and no case of navicular disease in the hind-feet has ever been 

 known. 



The navicular bone does not exercise any pulley function in con- 

 nection with the perforans tendon, such as has been usually de- 

 scribed — that is, if by the use of the term ' pulley ' it is intended 

 to convey the impression that some mechanical advantage is ob- 

 tained. It is true that by passing beneath the navicular bone the 

 direction of the pull of the tendon is altered, but no mechanical 

 advantage is thereby derived. The perforans tendon at its insertion 

 spreads out fan-shaped, and is attached over a considerable semi- 

 lunar surface of the pedal bone ; so extensive is this attachment that 

 it is erroneous to believe the tendon plays over the navicular bone. 

 It is a fact that movement occurs between the tendon and the 

 bone, but the tendon is passive, while the yielding of the navicular 

 bone under the influence of the body weight is the active agent. 

 It is interesting to observe the direction in which the largest amount 

 of friction occurs between these two surfaces. Reasoning from the 

 position of the parts, one would think it occurs at the moment 

 the foot comes to the ground ; but if the eroded tendon of navicular 

 disease be examined, it will be observed that the fibres are all stripped 

 upwards, and rarely or never downwards. This points to the 

 greatest friction occurring, not when the bone yields under the 

 weight, but when it returns to its place as the body, under the 

 influence of the flexor tendon, passes over the foot. The frequency 

 with which the middle of the ridge of the navicular bone, and the 

 area on either side of it, are affected with disease points to this part 

 as being the seat of the largest amount of pressure. 



Lateral Cartilages. — Attached to the heel of each pedal bone is 

 a large curved plate of cartilage, in parts fibrous, in others hyaline 

 in nature. So extensive is this plate that it reaches high above 

 the margin of the hoof — i.e., outside the foot in an upward direc- 

 tion as far forward as the coronet and as far back as the heel 

 (Fig. 219). There is no other structure in the body with which this 

 arrangement can be compared : a bone possessed of two large car- 

 tilaginous wings is a something peculiar to the foot. The use of 

 these cartilages is intimately connected with the main principles of 

 the physiology of the foot, to be dealt with later. 



Plantar Cushion. — Placed between the two plates of cartilage just 

 spoken of is a large somewhat pyramidal-shaped body known as the 

 plantar cushion (Figs. 220, 7, 7 ; 221). In appearance it resembles a 

 fibro-fatty mass, composed of interlacing bands, pale yellow in colour, 

 almost destitute of bloodvessels, firm to the touch, yet yielding in 

 its nature. 



Mettam* has shown that, though the plantar cushion to the naked 

 eye is fibro-fatty, its microscopical characters show it to be mainly 

 tendinous in structure, the fibres being disposed in bundles running 

 in different directions ; uniting the bundles is a connective tissue, in 

 the meshes of which fat is found in islets, and not abundant, as one 

 of the older names of this body implied, f There is only a small 

 amount of elastic tissue present. The position of the plantar cushion 

 in the foot is at the posterior half, between the cartilages, rising up 



* ' The Development and Histology of the Hoof,' etc., by Professor A. E. 

 Mettam, B.Sc, M.R.C.V.S., Veterinarian, 1896. 



f Nevertheless, W. C. Spooner, of Southampton, writing in 1840 

 ('Treatise on Foot and Leg of the Horse '), said analysis showed that 

 the plantar cushion contained no fat in its composition. 



