SOFT STRUCTURES OF THE FOOT. 549 



the base, and are secreted from the papillae on the surface of the 

 sensitive frog. The fibres of the coronary frog-band are very 

 fine, and are continued by a delicate layer of fibres, which covers 

 the external surface of the wall. Horn is tough, elastic, flexible, 

 and admirably adapted for protecting delicate parts ; it is secreted 

 in the form of cells, which become elongated. The colour of the 

 hoof varies, but it generally corresponds in depth with that of 

 the neighbouring skin. It is an acknowledged fact, that white 

 hoofs, like white skin, are more delicate, porous, and prone to 

 disease than dark ones. 



SOFT STRUCTURES OF THE FOOT. 



The parts contained within the hoof are the os pedis, os navi- 

 culare, the distal extremity of the os coron^e, the ligaments by 

 which these are connected together, the insertion of the extensor 

 pedis and flexor perforans tendons, and the vessels and nerves. 

 These have been described in their respective sections ; but there 

 are also certain structures proper to the foot which it is necessary 

 to describe here — viz., the lateral cartilages, sensitive frog, 

 coronary band, sensitive laminae, and sensitive sole. 



The Lateral cartilages are two thin plates composed of 

 hyaline cartilage in the middle, partaking more of the nature of 

 fibro-cartilage towards the borders. They are of an irregularly 

 quadrangular form, and surmount the wings of the coffin bone ; 

 each presents two surfaces and four borders. The external 

 surface is convex, the internal one concave, presenting numerous 

 vascular grooves, and is continuous postero-interiorly with the 

 sensitive frog. The superior border is thin, generally convex, 

 and pierced by several foramina for vessels to pass to the frog ; 

 the inferior border is attached to the wing of the os pedis 

 posteriorly, it is inflected inwards, and is continuous with the 

 inferior surface of the sensitive frog ; the posterior border is 

 slightly convex, and has a direction obliquely downwards and 

 backwards, joining the superior and inferior borders, being 

 separated from the former by an obtuse angle ; the anterior 

 border, also directed obliquely downwards and backwards, be- 

 comes intimately blended with the anterior lateral ligament of 

 the coffin joint. The anterior borders of the two lateral cartilages 

 are united by fibrous expansions from the extensor tendon and 

 the above-mentioned lateral ligament. 



