810 



THE APPARATUS OF THE SENSES. 



CONSTITUENT ELEMENTS 

 OF THE WALL. 



Pigmentary-corpuscles exist in the substance of the coloured horn, 

 and are disposed singly, or in small masses, in the epithelial cells of the 

 intertubular substance. The presence of these corpuscles has been denied, 

 and the coloration has been attributed to a greater 

 condensation, at certain points, of the epithelial 

 elements. Fine pigment-granules are disseminated 

 in the cells, but it is evident that beyon/1 these there 

 are at different points pigment-corpuscles ; for, after 

 treating a section of coloured horn with soda, the 

 epithelial elements are distended, become pale, and 

 disappear, leaving, however, here and there, masses 

 of black granulations. These pigmentary-corpuscles 

 are absent in white horn. 



DEVELOPMENT OF THE HOOF. The hoof being a 

 dependency of the epidermis, is developed like it: 

 that is, by the incessant formation of cells in the 

 layer that corresponds to the rete mucosum, at the 

 expense of the plasma thrown out by the numerous 

 'A fjjjBfi [)Wk vessels in the keratogenous membrane. The velvety 



| fjHMM W tissue forms the sole and frog ; the perioplic ring the 



periople, and the coronary cushion the wall. In 

 these different parts, the epithelial cells multiply, 

 and become flattened in layers parallel to the surface 

 that secretes them, and in proportion as they recede 

 a, Horn-cells; 6, Horn- from that surface; so that the wall grows from its 

 fibre from the hoof of superior to its inferior border, and the other two parts 

 a new-born foal show- f th hoof from their internal to their external face. 

 intf the vertical disposi- m , ... . . , , . , ., 



tion of the cells. The villosities of the coronary cushion and the 



velvety tissue are the organs around which the epi- 

 thelial lamellse 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 leafy 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 laminse. 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 

 lamina is based on comparative anatomy, on the presence of some lon- 

 gitudinal tubes in the horny laminae, and on pathological observations. 



When the podophyllous tissue is inflamed, whether or not it is 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 

 M. 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 

 laminaB. In this horn, produced by the vascular laminae only, there are 

 never observed between these latter the horny plates of cells sharp and 



