X] THE ORGANS OF LOCOMOTION 129 



which is the final propelling part of the foot the friction is greatest. 

 The base being in reality the inflected portion of the wall are, 

 like the rest of the wall, intended to bear weight. The inflection 

 admits of there being room for the elastic portion of the foot (the 

 plantar cushion). 



The sole is normally concave. As a practical matter it cannot 

 be too thick. While the wall can go on growing indefinitely the 

 sole cannot, since after a certain degree of groAvth the horn fibres 

 become disintegrated and scales are shed. The junction of the 

 sole and wall is marked by a white line where the horn is softer. 

 The function of the horny sole is to provide protection for the 

 sensitive structures within ; its concave surface does not adapt it 

 for carr^-ing weight, exceptmg at the edge where it is connected 

 with the wall. 



The foot-pad or frog is moulded on the plantar cushion. The 

 horn of which it is composed is markedly elastic and contains 

 a relativeh' large amount of moisture. Like the sole it casts out 

 scales. The frog serves as an anti-concussion mechanism, for the 

 impacts it receives are transferred to the plantar cushion and 

 thence to the lateral cartilages and wall of the foot which are 

 stretched outwards. The frog normally comes into contact with 

 the ground, and if this is not the case it is liable to disease. In 

 hot drj' climates the horny parts of the foot, especially of the 

 frog, tend to become excessively hard and to cause lameness 

 from bruising the underlying sensitive structures. 



The Structure of the Horn. The horn of all parts of the hoof 

 consists essentially of keratinised epithelial cells which are 

 spindle-shaped, oblong, or irregular. They contain nuclei, 

 granular matter, and verj' often pigment, and are united by a 

 cement substance. The horn substance is soluble in caustic 

 alkali, and consequenth^ horses should not to be allowed to 

 stand in their own urine. 



The horn structure in the progress of its development acquires 

 canals or tubes which are used as a system of irrigation through 

 which the hoof is supplied with the moisture necessar}' for pre- 

 serving a proper elasticity. Without this moisture the horn 

 substance crumbles. Evaporation from the substance is regulated 

 by the periople as already mentioned, but nevertheless it is con- 

 stantly taking place in some degree. In shoeing it is important 

 to avoid injury to the periople as much as possible. The elasticity 



