HORSESHOEING. 



67 



Fig. 39. 



possesses so very little resistance (strength) that it is often 



found crumbling or even absent in 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 nist from 



the nails. The white line is 



very important, since it 



serves as the point from 



which we judge of the 



thickness of the wall, and 



because the horseshoe nail a homy frog cut vertically and lengthwise 



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

 Should penetrate it. stay; c, median lacuna of frog, which at c', is 



_,, _, , -T-<* o^ 7 overlaid with superposed layers of horn. 



The Frog (Figs. 31, h, 

 35, k, I, 38 and 39), secreted by the velvety tissue covering 

 the plantar cushion and presenting almost the same form 

 as tlie latter, lies as a wedge between the bars and between 



Fig. 40. 



Longitudinal section of the wall magnified. The dark stripes parallel and close together 

 are horn-tubes; the lighter surface between the tubes represents the intertubular horn. 

 Notice that the horn-tubes are of various diameters. The space between a and b represents 

 the small tubes 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 

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

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

 and outward towards the wall. 



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 of the 



