THE APPENDAGES OF THE SKIN. 911 



lunar ridge and the insertion of the perforans tendon, with which the plantar 

 cushion mixes its fibres at this part. 



The lateral borders are wider behind than before, in consequence of the 

 wedge-like shape of the whole organ ; they are continuous with the inner face 

 of the lateral cartilages, as already indicated in describing the latter. 



The organization of the plantar cushion differs much from that of the carti- 

 lages. It has for its base a fibrous structure, continuous with that which consti- 

 tutes the fundamental framework of these ; this structure is very dense towards 

 the infero-posterior part of the organ, and becomes gradually looser as it leaves 

 this region ; the meshes it contains are filled with a yellow pulp composed of 

 fine, elastic, and connective fibres, in the midst of which some adipose cells are 

 found. (I have attentively examined this yellow pulp, and can perceive that it 

 is essentially constituted by adipose tissue). Numerous blood-vessels and nerves 

 complete this structure. 



B. The Keratogenous or Subcorneus Membrane.— The keratogenous 

 membrane envelops the extremity of the digit, by spreading over the terminal 

 ■expansion of the tendon of the extensor pedis, through the medium of a fibrous 

 fascia — a dependency of the lateral cartilages ; and also over the inferior moiety 

 ■of the external face of these cartilages, the bulbs of the plantar cushion, pyramidal 

 l)ody, anterior part of the plantar face of the third phalanx, and over the anterior 

 •surface of the same bone. It covers all these parts like a sock, and the hoof 

 ■encloses it, as a shoe does the human foot. 



This membrane becomes continuous with the skin of the digital region, at a 

 circular liue that intereects the middle portion of the second phalanx, and in- 

 clines obliquely downward from behind to before. Below this line, in front and 

 laterally, the subungular tissues form a semi-cylindrical protuberance, covered 

 with vilH, and designated the " bouiTelet." (This elastic prominence has received 

 several most inappropriate names from English farriers and hippotomists, such 

 as " coronary ligament," " coronary substance," " cutiduris," etc. From its 

 function, structure, situation, and its analogy to the plantar cushion, I have 

 designated it the " coronary cushion.") 



On the plantar cushion and the lower face of the pedal bone, this membrane 

 becomes a villous tunic — the velvety tissue — which is continuous, towards the 

 bulbs, with the extremities of the coronary cushion. 



The portion spread over the anterior face of the third phalanx constitutes the 

 laminal {podophyllous) or "leafy tissue," so called because of the laminae or 

 parallel leaves seen on its surface. 



The three regions of the keratogenous apparatus will be successively studied. 



1. Coronary Cushion. — This part is the matrix of the wall of the hoof, 

 and is lodged in a cavity excavated at the upper border of this part of the horny 

 case. It forms, according to the expression employed by M. Bouley, a rounded 

 prominence, which projects like a cornice above the podophyllous tissue. 



Its inferior border is separated by a white zone from the upper extremity of 

 the laminae, which constitute this boundary. 



The superior border is limited by a slightly projecting margin named the 

 perioplic ring, because it secretes the horn of the periople. Between this margin 

 and the cushion is a sharply defined groove. 



The extremities, narrower than the middle portion, on arriving at the bulbs 

 of the plantar cushion, bend downwards into the lateral lacunae of the pyramidal 

 body, where they become confounded with the velvety tissue. 



