THEUSH. 165 



or careless observer may, not unlikely, assume the ailment to be 

 navicular disease. When thrush is caused by wet, the frog becomes 

 soft and pulpy. In the absence of moisture acting on the feet, 

 there may be little or no discharge from the cleft of the frog, 

 which will be then more or less filled with soft, degraded horn of 

 a cheese-like consistence. In this case, the frog will be more or 

 less shrivelled up. The presence of the disease is always accom- 

 panied by an offensive and peculiar smell. 



Fig. 48. — Foot with contracted heels and thrush. 



Pressure on the frog causes an increased growth of epithelium 

 (p. 154). Hence, when the frog is exposed to continual pressure, 

 it becomes strong and well developed ; but, if deprived of this 

 natural stimulus, it becomes diseased and contracted in size. The 

 human skin of the hands and feet is as dependent on continued 

 pressure for its strength and hardness, as is the frog of the horse. 



Horses worked (as in riding schools), or stabled, on tan, are. 

 very liable to thrush from the tan becoming balled in the feet, 

 thereby causing the frog and sole to get heated by stopping 

 evaporation of moisture from them. The ill effects of the tan, in 



