528 ANATOMY AND PHYSIOLOGY OF THE HORSE’S FOOT. 
far as I am aware ; but it might be suggested that its soft- 
ness and flexibility must tend to avert disunion between the 
lower margin of the wall and the border of the sole, or even 
fracture of these — an accident which would be likely to 
occur were these two comparatively rigid and dense pieces 
directly joined to each other without the intervention of the 
white line of horn. Its presence may also permit that in- 
dependence and dissimilarity of growth that exists between 
the sole and wall, and which constitutes so marked a feature 
in the physiology of this organ. 
The “ frog,” and what is termed the “ periople,” may be 
said to possess the same histological characters as the other 
parts of the hoof : they are fibrous in structure, the cells 
composing them have the same vertical and horizontal direc- 
tions, the fibres are generally parallel to each other, and on 
the whole follow the same course as those of the sole and 
wall. In other respects, however, they are different. The 
fibres, instead of being quite straight, are, particularly in the 
frog, more or less wavy and slender, the cells composing 
them have very delicate walls, and contain little keratin and 
earthy constituents, but more organic matter than those of 
the wall and sole ; they are very readily acted upon by 
alkalies, which rapidly dissolve them. Consequently, when 
treated with caustic potash they do not exhibit the play of 
colours by the polariscope which the sole or wall similarly 
acted upon by this reagent, for the same period, would. In 
Canada halsam or turpentine the only differences to be per- 
ceived by polarized light are the fainter tints of the frog and 
periople, and each fibre, in a transverse section, showing a 
dark cross not unlike that displayed by the cylinders of 
rhinoceros horn or whalebone, or starch granules, when 
viewed by this light. Another peculiarity this soft horn 
possesses is the facility with which it becomes stained by 
carmine, when compared with that of the wall and Sole, 
which is extremely refractory to the imbibition of this colour- 
ing matter, except the interior of the tubular portion of the 
fibre in the immediate vicinity of, or immediately in contact 
with, the villi. By means of the carmine fluid we can trace 
the presence of the periople in some hoofs at a distance below 
the middle of the wall, where its presence is otherwise indis- 
tinguishable to the naked eye, and nothing is observed but a 
smooth, glistening, varnish-like appearance, should the hoof 
not have been tampered with by the farrier’s rasp. 
'/he periople (fig. 2, plate II), in several of its features, 
corresponds to, or rather is the representative of, the inflected 
skin at the root of the human nail. It commences almost 
