48 



to be of a finer and more delicate structure than those of the 

 sensitive sole. They also take the same oblique direction 

 downwards and forwards as those of the sensitive sole. View- 

 ing the internal aspect of the hoof, a long, deep, triangular 

 fissure will be seen, with a small eminence springing from its 

 medial line, near the basal end of the triangle. In and over 

 this fissure, and adjacent thereto, the sensitive frog reposes. 

 This sensitive frog may also be regarded as an interperiosteal 

 pad or cushion, only, compared to the others, it is of greatly 

 magnified dimensions. It fills up the whole of the posterior 

 space of the foot not occupied by other substantial tissues. 

 This, the principal spring-cushion of the foot, as to size, is com- 

 posed of two kinds of ligamentous tissue — the white fibrous 

 and the yellow elastic. The mass of the frog-cushion is com- 

 posed of the same material as constitutes the sub-cutaneous 

 tissues, usually designated connective or areolar tissues. To- 

 wards the posterior portion of the foot these tissues are trans- 

 formed into a vast fibro-cartilaginous cushion, otherwise known 

 as the sensitive frog. Its eminently elastic properties are due 

 to its yellow elastic constituent, which predominates most in 

 the upper portion of the pad that lies in close contiguity to the 

 perforans tendon and the inferior portion of the coronal bone. 



CHARACTERISTIC STRUCTURE OF THE FROG. 



Another remarkable feature of the constitution of the frog- 

 cushion, which only the microscope could reveal, is the manner 

 in which these fibrous materials of the sensitive frog are dis- 

 posed. There is no apparent order in their disposition, but 

 they are interwoven with each other in every conceivable direc- 

 tion, thus forming a dense areolated mass of tissue. These 

 areolae are exceedingly minute, and it is within these that the 

 yellow elastic fibres called " nuclear " are discovered to exist. 

 It is this variety of fibre that gives elastic properties to an oth- 

 erwise inelastic substance. The chief characteristic, therefore, 

 of the frog-cushion is firmness, combined with lightness and 

 elasticity, it being endowed with the latter property in every 

 conceivable direction. 



