THE SENSITIVE SOLE AND FROG. 73 



sole carries a very well-marked venous plexus, and produces 

 the horn of the sole. 



5. The Sensitive Viioa. 

 (Fig. 48, c.) 



The sensitive frog is that portion of the corium which covers 

 the plantar cushion. It clothes the groove of the frog (d), 

 and insensibly unites in the neighbourhood of the bulbs (e) 

 with the perioplic ring, so that no distinct boundary can be 

 drawn between them. In general, the sensitive frog is not 

 so well supplied with vessels as the sensitive sole, and, therefore, 

 presents a lighter colour. The papillae on the lower surface 

 are somewhat longer than those at the sides and base. The 

 sensitive frog produces the horny frog. 



Under the term sensitive frog is often included the plantar 

 cushion, together with its horn-secreting covering. This, how- 

 ever, is anatomically incorrect. The sensitive frog clothes the 

 plantar cushion in the same way as the sensitive sole clothes 

 the under surface of the os pedis, and the sensitive wall the 

 laminal surface of this bone. The plantar cushion and the 

 horn-secreting surface are entirely different structures. The 

 former is not a mere thickening of the latter, but a tissue, 

 formed of elastic and fibrous components, which fulfils a special 

 physiological function. 



B. THE HORXY STRUCTURES. 



The collective masses of horn produced by the active epithelium 

 of the foot are termed the hoof (fig. 44). This presents the 

 appearance of a capsule enclosing the, lower end of the limb, 

 and comports itself towards the latter much as a shoe to the 

 human foot. The connection between the horny capsule and the 

 corium is so intimate that in the healthy tissues the two can 

 never be dissected apart. Only in certain diseases of the foot 

 do the sensitive and insensitive parts of tlie foot become more 

 or less disunited. Occasionally, and in very severe cases, the 

 hoof may, however, part from the corium, but after death decom- 

 position very soon loosens the connection, the line of separation 



