166 DISEASES OF THE HORSE'S FOOT 



coronet is hot and tender to the touch, and a sensation of 

 warmth is sometimes conveyed to the hand by the horn of 

 the surrounding parts of the wall. It is hardly necessary 

 to say that, with accompanying conditions such as these, 

 the sand-crack is a deep one. 



Where the lameness is but slight, we may attribute it 

 almost solely to the pain occasioned by the mere wounding 

 of the keratogenous membrane, and to no very extensive 

 inflammatory changes therein. By some authorities this 

 is said to be due to the pinching of the sensitive structures 

 between the edges of the fissure in the horny covering. In 

 our opinion, however, pinching does not occur unless in- 

 flammatory exudation into the sensitive structures adjoin- 

 ing the crack has led to sufficient swelling to cause them to 

 protrude. In other words, the movements of the horny 

 box, communicating themselves to the structures beneath, 

 and so occasioning movement in the wounded keratogenous 

 membrane, are quite sufficient to give rise to the lameness 

 without actual pinching of the structures implicated. 



The severity of the lameness will vary with the rapidity 

 of the gait, and with the character of the road upon which 

 the animal is made to travel. For instance, many animals 

 in which the lameness is imperceptible at a walk become 

 ' dead ' lame at a fast trot. It is sufficiently explained 

 when one remembers the greater movements of expansion 

 and contraction of the posterior parts of the wall brought 

 about by the increase in the rate of progression. The same 

 animal, too, will go distinctly more lame upon a hard than 

 upon a soft surface. 



In like manner the lameness from toe-crack also varies in 

 degree with the rate of progression and the character of the 

 travelling, though not to such a noticeable extent as in the 

 lameness from quarter-crack- A greater variation may in 

 this case be brought about by moving the animal on 

 ascending and descending ground. Descending an incline, 

 with a more than ordinary share of the body-weight thus 

 thrown upon the heels, the lameness is most marked. The 

 reason would appear to be that the greater expansion of 



