808 th:e appabatus of the senses. 



point wedged in tlie re-entering angle comprised between the two portions 

 of the inner border of the sole. 



In the Ass and Mule, the hoof is always narrower, laterally, than that of 

 the horse ■ the wall is always higher and thicker, the sole more concave, 

 the frog smaller and deeper seated at the bottom of the excavation formed 

 by the sole, and the horn is much more hard and resisting. 



(The angle of the wall of the hoof in front varies from 50° to 56°, 

 though usually erroneously stated to be 45°. The inner face of the wall, 

 at the middle of the toe, and in a line with the frog-stay, frequently shows a 

 more or less salient and conical prominence — base towards the lower margin 

 of the wall — which corresponds to a vertical depression in the os pedis. 

 Vallada imagined that this projection served to unite the wall and sole more 

 closely, but it is far more probable that its function is the same as that of 

 the frog-stay — to maintain the position of the os pedis, and prevent its 

 rotation within the hoof. I have, therefore, named it the " toe-stay.") 



Stbtjctube of the Hoop-hokn. — The structure of the horn has been the 

 object of a great number of researches ; Gurlt, Delafond, Bouley, Gourdon, 

 and Ercolani ' have given descriptions of it, and we have also some details to 

 add to the labours of these authorities. 



The horny substance constituting the hoof of Solipeds, has a fibrous 

 appearance ; this is most conspicuous in the wall, less apj)arent in the frog 

 and deeper portions of the sole, but impossible to distinguish in the 

 superficial layer of the latter, where the disintegration continually taking 

 place separates the horn in scaly fragments of varying thickness and 

 extent. The consistence of the horn is always less in the frog than in the 

 sole and wall. Its tint is in some hoofs black, in others white, and in 

 others again a mixture of these two. The inner face of the wall, however, 

 is never black ; and when the lower part of the limb is partially or wholly 

 white, we may be sure that all the thickness of the wall will either be white 

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



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

 always exhibits the same characters ; everywhere it is perforated by cylin- 

 drical canals whose upper end 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 laminae. All are rectilinear, with the exception of those of the frog, 

 which are some'^hat 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 the 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 

 papillee of the keratogenous membrane at their superior extremity, these 



' (The researches of Professor Eawitsch must not be omitted. They will be found in 

 Volume xxviii. of the ' Magazin fiir Thierheilkunde,' and also in a little hrochure 

 entitled ' Veber den feineren Bau und das Wachsthum des Hu/horns.' Berlin, 1863. 

 Leisering must likewise be referred to. My own researches are published in the 

 ' Veterinarian' for 1871.) 



