548 



iESTHESIOLOGY. 



part of the lateral surfaces of the bars and the frog are deep 

 triangular hollows, called the commissures of the frog. The 

 base or posterior extremities constitute the heels or bulbs of the 

 frog ; these are two round flexible and elastic eminences, formed 

 by the two extremities, and separated by the cleft. They cover 

 the two angles of inflection of the walls, and are continued round 

 the external part of the superior surface of the wall in the form of 

 a band, which is the coronary frog band ; it is from the inferior 

 border of this that the thin covering of the hoof is continued. 

 r)- _^ ._> The summit or toe of the frog is the anterior 

 pointed portion, which is inserted into the 

 narrow part of the notch in the sole. 



HORN TISSUE. 



The horn of the hoof presents a fibrous 

 appearance, very distinct in the wall, less so 

 in the deep-seated parts of the sole and frog, 

 and scarcely traceable in the superficies of 

 the sole, where there is a continual disintegra- 

 tion or separation of the horn in layers more 

 or less extensive. It consists of minute 

 cylindrical tubes or fibres, which have a direc- 

 tion obliquely downwards and forwards, and 

 are arranged in parallel rows, forming layers, 

 placed one over .the other, and internally 

 lamellae or plates, the whole united together 

 by an amorphous or intertubular substance. 

 Besides tha fibres, plates, and intertubular 

 substance, there i^s always more or less pig- 

 mentary matter, which consists of colour-cells 

 distributed amongst the concentric plates or 

 layers forming the walls. The horny fibres of 

 the sole have the same obliquity as those 

 of the walls, are secreted from the villi on 

 the sensitive sole, and finally united to the wall 'round the 

 outer circumference ; they receive a few villi from the inferior 

 extremity of each sensitive lamina. 



The horn of the frog difi*ers considerably from that of both 

 wall and sole. The tubes are much finer and softer, and have a 

 flexuous arrangement They are united to those of the walls a^ 



Fiu. 213. 

 Transverse section of 

 the horn of the wall 

 magnified, a, Periople : 

 b, Horn tissue : c c, Homj' 

 laminae ; d d. Sensitive 

 laminae. 



