HORSESHOEING. 



51 



Fig. 33. 



places. The visible part of the white line is usually of a grayish- 

 black color, owing to the working in from below of dirt and 

 liquid manure, and to staining by rust from the nails. The white 

 line is very important, since 

 it serves as the point from 

 which we judge of the thick- 

 ness of the wall, and be- 

 cause the horseshoe nail 

 should penetrate it. 



The Prog (Figs. 27, h, 29, 

 k, I, 32, and 33), secreted by 

 the velvety tissue covering 

 the plantar cushion and pre- 

 senting almost the same form 

 as the latter, lies as a wedge 



between the bars and between the edges of the sole just in front 

 of the bars, with both of which structures it is intimately united. 

 Its horn is quite soft and very elastic. The median lacuna or cleft 



Fig. 34. 



A homy frog cut vertically and lengthwise 

 through its middle : a, upper surface ; 6, frog- 

 stay ; c, median lacuna of frog, which at c' is 

 overlaid with superposed layers of horn. 



Longitudinal section of the wall magnified The dark stripes parallel and clow together are 

 horn-tuhes ; the lighter surface hetween the tuhes represents the intertubular horn. Notice 

 that the horn-tubes are of various diameters. The space hetween a and 6 represents the small 

 tuhes of the outer, darker horn of the principal (middle) layer of the wall ; the space between 

 b and c the lighter, inner horn of the wall ; c, d, the horn separating the wall proper from the 

 homy leaves ; d, e, the homy leaves (keraphyllous tissue), on which can be seen fine, parallel, 

 vertical stripes ; in the horn-leaf at /, / are seen fissures passing obliquely upward and out- 

 ward towards the wall. 



of the frog (Fig. 27, I) divides it into two branches (Fig. 27, i\ 

 which pass backward and outward into the horny bulbs (Fig. 27, 



