THE MATRIX OF THE HOOF 



771 



The digital or plantar cushion (Torus digitahs) is an important part of the 

 elastic apparatus of the foot. It is a wedge-shaped mass which overhes the frog. 

 It presents for description four surfaces, a base, and an apex. The superior surface 

 faces upward and forward and is connected with the distal fibrous sheath of the 

 deep flexor tendon. The inferior surface, covered by the matrix of the frog, is 

 moulded on the upper face of the frog. The lateral surfaces are related chiefly to 

 the lateral cartilages; inferiorly the cushion is closely attached to the cartilages, 

 but higher up a rich venous plexus intervenes. The base, situated posteriorly, is 

 partly subcutaneous, and is divided by a central depression into two rounded 

 prominences termed the bulbs of the plantar cushion. The apex lies under the 

 terminal part of the deep flexor tendon. The cushion is poorly supplied with ves- 

 sels. It consists of a feltwork of fibrous trabecular and elastic fibers, in the meshes 

 of which are masses of fat. The bulbs are soft and loose in texture and contain a 

 relatively large amount of fat, but toward the apex the cushion becomes denser and 

 more purely white fibrous in structure. Branched coil glands occur chiefly in the 

 part of the cushion which overlies 

 the central ritlge of the frog. Their 

 ducts jjursue a slightly flexuous course 

 through the matrix and pass in a spiral 

 manner through the frog. Their secre- 

 tion contains fat. 



Vessels and Nerves. — The matrix 

 is richly supplied with blood by the 

 digital arteries. The veins are valve- 

 less, and form remarkable plexuses 

 which communicate freely with each 

 other and are drained by the digital 

 veins. The lymph vessels form sub- 

 papillary plexuses in the matrix of the 

 sole and frog, and a wider-meshed 

 plexus at the base of the plantar 

 cushion. A considerable lymph vessel 

 lies in the attached edge of each of the 

 lamina?. The nerves are branches of 

 the digital nerves; some fibers end in 

 lamellar corpuscles and end-bulbs. 



The ergot is a small mass of horn 

 which is situated in the tuft of hair at 

 the flexion surface of the fetlock. It 



is the vestige of the second and fourth digits of extinct equida^, and hence is absent 

 in cases in which these digits are developed. A small tendinous band, 3 to 5 mm. 

 in width, extends downward and slightly forward from the fibrous basis of the 

 ergot on each side, crosses over the digital vessels and nerves very obliquely, and 

 blends below with the digital fascia and the plantar cushion. It is known as the 

 ligament or tendon of the ergot. 



The term chestnut is apphed to the masses of horn which occur on the inner 

 surface of the forearm about a handbreadth above the carpus, and on the lower 

 part of the inner face of the tarsus. They have an elongated oval form and are 

 flattened. They are regarded usually as vestiges of the first digit. That of the 

 hind limb is absent in the donkey and very small in the mule. 



These horny vestiges are quite variable in form and size and are correlated with the fineness 

 or coarseness of the integument in general. The supracarpal chestnut is about 132 to 23^ mches 

 long, oval in outline, the proximal end being pointed; it overlies the flexor carpi mternus at a 

 quite variable distance above the carpus, and hence should not be used as a .surgical landmark. 

 The tarsal chestnut lies at a point behind the lower part of the internal lateral ligament of the 



Fig. 582. — Cross-section of Digit op Horse, through 

 Distal End of Second Phalanx. 

 a. Second i)halanx; b, deep flexor tendon; h' , 

 digital synovial sheath; c, plantar cushion; d, lateral 

 cartilage; e, skin; /, hoof; g, anterior extensor tendon; 

 h, cavity of coffin joint; ?', suspensory ligament of navi- 

 cular bone; k, lateral ligament of coffin joint; /, digital 

 artery; m, digital nerve. (After EUenberger, in Leiser- 

 ing's Atlas.) 



