918 THH SENSORY APPARATUSES. 



or wholly white, we may be sure that all the thiclmess of the wall will either be 

 white at corresponding points in the former, or entirely so in the latter. 



Except in the keraphyllous tissue, the minute structure of the hoof-horn 

 always exhibits the same characters. Everywhere it is perforated by cylindrical 

 canals, the upper end of which is funnel-shaped, and these contain the papillae of the 

 matrix, whether they belong to the coronary cushion or velvety tissue ; while the 

 lower end reaches the inferior border of the wall, or lower face of the sole and 

 frog, according to their situation. It is rare to find them in the horny laminas. 

 All are rectilinear, with the exception of those of the frog, which are somewhat 

 flexuous ; and all have the same oblique direction downward and forward, 

 following the inclination of the anterior portion of the wall. They are, therefore, 

 almost exactly parallel to each other, not only in the same, but in two different 

 regions. Their diameter varies considerably, though the smallest are always those 

 of the periople ; in the wall, they are smaller as they approach the outer surface. 



These tubes are not mere canals hollowed out of the horny substance ; on the 

 contrary, they have very thick walls which are formed of numerous concentric 

 layers, one within the other, and the horny tissue connecting them has not the 

 same apparent stratiform disposition. Filled by the papillae of the keratogenous 

 membrane at their superior extremity, these canals are not empty for the remainder 

 of their extent ; but contain a particular white substance, which is so opaque 

 that it appears of a fine black hue when examined as a transparency in the micro- 

 scope. This matter is not deposited in a uniform manner in the canals, but 

 irregularly, looking like a knotted cord or a necklet of beads ; and where it does 

 exist, it does not always exactly fill the calibre of the tube, an interval being 

 observed between the inner face of the latter and the intra-tubular deposit. 

 Sometimes it is seen outside the canals, among the concentric lamellae, and even 

 in the horny intertubular substance 



If we are desirous of completing our knowledge of the minute organization 

 of the hoof -horn by studying the anatomical elemmts constituting it, we shall find 

 that it is formed of epithelial cells belonging to the kind most widespread in 

 the economy — pavement epithelium. These horn epithelial cells are very thin, 

 pale, polygonal, and generally oblong, have sharply defined borders and finely 

 granular faces, sometimes showing a nucleus containing a single or multiple 

 nucleolus. The nuclei sometimes occupy the centre, at other times another part 

 of the cells — even their margins ; and they also contain pigment-granules more 

 or less coloured and numerous. Acetic acid acts very slowly on them, and is 

 limited to making them more transparent. Potassium and soda at first softens, 

 then distends them, causing their granulated aspect to disappear, and rounding 

 their contours ; afterwards, they become quite diaphanous, and finish by being 

 completely dissolved 



Examined in their reciprocal relations, these epithelial cells are seen not to 

 be agglomerated comfusedly together, but are, on the contrary, disposed in a 

 regular manner, forming a real framework that wonderfully concurs in assuring 

 solidity and flexibility. In the walls of the tubes we see them arranged 

 horizontally around the canal, and stratified from within to without, so as to 

 form successive concentric layers. In the intertubular horn they are disposed 

 differently, their stratification being no longer parallel to the direction of the 

 tubes, but perpendicular to it, and piled upon each other in the intervals 

 separating the latter. This change of direction does not occur suddenly ; at 

 thf limits of the tubes epithelial cells are seen lying obliquely. 



