920 THE SENSORY APPARATUSES. 



that surface ; so that the wall grows from its superior to its inferior border, and 

 the other two parts of the hoof from their internal to their external face. 



The villi of the coronary cushion and the velvety tissue are the organs 

 around which the epithelial lamellae are grouped, and their presence determines 

 the tubular structure of the horn ; their function is completed by the exhalation 

 of a particular fluid that maintains the flexibility of the hoof, and, probably, 

 by the development on their surface of the irregular cells which cluster in the 

 interior of the tubes. 



The laminal tissue, in a normal condition, does not concur to any extent in 

 the development of the wall. The cells covering it are multiplied in describing 

 a downward and forward movement ; and though they are certainly applied to 

 the inner face of the wall, yet they do not constitute the horny lamiuis. The 

 latter are formed on the coronary cushion, at the commencement of the vascular 

 laminae, and they descend with the wall in gliding along the surface of the layer 

 of cells separating them from the latter ; this downward movement is facilitated 

 by the multiplication, in the same sense, of these cells. This opinion as to 

 the function of the vascular laminae is based on comparative anatomy, on the 

 presence of some longitudinal tubes in the horny laminae, and on pathological 

 observations. 



When the podophyllous tissue is inflamed — whether or not it be exposed — its 

 latent activity is quickly manifested, and it rapidly throws out a large quantity 

 of hard consistent horn, traversed by tubes which, according to Gourdon, are 

 directed obliquely backwards. These tubes are more irregular than those of the 

 normal wall, are disposed in parallel series, and are formed around the villo- 

 papillae developed on the free border of the laminae. In this horn, produced by 

 the vascular laminae only, there are never observed between these latter the 

 horny plates of cells — sharp and distinct in the midst of the other cells, as in 

 those of the wall formed by the coronary cushion. 



The horn thrown out on the surface of the podophyllae, immediately after 

 the removal of a fragment of the wall, is not a definitive horn, but must 

 be replaced by that from the coronet. This substitution is complete ; as a 

 microscopical examination proves that the wall which descends from the cushion, 

 and is furnished with horny laminas, passes beneath the provisional wall, and 

 glides downward — by the combined action already mentioned — over the surface 

 of the soft cells of the vascular laminae. As soon as the latter are covered by 

 the proper wall, their marginal papillae become atrophied, and they again assume 

 the limited function pertaining to their physiological condition. 



(The description of the disposition of the epithehal cells given by Chauveau 

 does not quite coincide with my own observations. As he correctly states, these 

 cells are formed in planes parallel with the surface that secretes them ; conse- 

 quently, around the papillae they are more or less vertical, while between them they 

 are horizontal. The walls of the tubes, or fibres, are therefore composed of cells 

 disposed in a vertical manner ; while in the inter-fibrous horny matter they 

 are arranged in the opposite direction. The loose nodulated contents of the 

 tube is composed of cells thrown off from the termination of the papilla, and 

 corresponds to the pith of feathers. The soft cells interposed between the 

 vascular and horny laminas are carried down to the lower margin of the wall, 

 where — with the elastic horn secreted by the papillae which terminate the former — 

 they constitute the peculiar light-coloured band, or " white line," which marks 

 the junction of the sole with the wall (Fig. 499, g). This intermediate band of 



