322 THE EXTERIOR OF THE HORSE. 



whether they belong to the anterior or the posterior biped : the external 

 side of the wall is constantly more inclined and more convex than the 

 internal ; also, the inferior border of the wall is always rounder and 

 stronger on this side than on the other. 



B. Properties and Mechanism of the Foot. 



The foot of the horse, such as we have described, is an organ which 

 enjoys certain properties inherent both to the nature and disposition 

 of the tissues which compose its internal parts and to these same 

 qualities possessed by the horny envelope. It is on account of these 

 properties that the foot can fulfil, with all the desirable perfection, the 

 important functions which devolve upon it. 



The Horn : Properties. The horn is a hard, compact, resisting, 

 elastic, and tenacious substance, which is softened by contact with 

 water or the influence of dampness, and hardens by evaporation. It 

 is very combustible, and gives origin, when burning, to abundant 

 fumes of a characteristic odor and a carbonaceous adherent matter 

 which protects the living tissues against the action of the heat. 



a. Origin. The diverse regions of the cutaneous envelope (kerato- 

 genous membrane, flesh of the foot) of the foot do not contribute in 

 the same manner to the formation of the horn. As our colleague, 

 M. Arloing, 1 judiciously remarked, some are keratogenous and others 

 keratophorous. Upon the first (coronary band, velvety tissue, perioplic 

 ring) is imposed the elaboration of the nail, properly so called ; upon 

 the second (podophyllous tissue) more particularly devolves the role of 

 support, consolidation, and union. Analogous to a sort of physiologi- 

 cal cement, the latter establishes the relation and adherence between the 

 horn and the living parts. Its production is represented for this pur- 

 pose by the white, horny laminse situated on the deep face of the wall, 

 to which we have made reference above under the name keraphyllous 

 tissue. 



Thus the wall, the sole, and the frog are formed by the keratogenous 

 regions of the ungual matrix. 



b. Structure. Under the microscope, the horn appears consti- 

 tuted by a great number of parallel tubes, rectilinear (wall, sole) or 

 undulating (frog), which originate from the vascular papillae of the 

 cutidure or the velvety tissue by enveloping them in the most intimate 

 manner. Altogether comparable to hairs of a large calibre placed side 

 by side, these tubes are composed of a multitude of horny (epithelial) 



i S. Arloing, Foils et Ongles, these d'agre"gation, 1880, p. 111. 



