SAND-CRACK. 375 



horny structures. The special history of the horny appendages 

 of animals consists therefore in the description of the form and 

 disposition of the surfaces from which they spring. Whereas 

 hairs have roots embedded in follicles, the horn tubes spring 

 from papillae, which stud a surface extended over bony or fibro- 

 elastic prominences." — (Gamgee.) 



Horn is often spoken of as " built of hairs firmly matted 

 together " (Barlow). The horn tubes are united together by 

 an inter-tubular substance composed of cells, and produced from 

 the surface between the papillae. The wall, consisting of horn 

 tubes and agglutinating inter-tubular substance, is secreted by 

 the coronary substance, and is naturally tough, but breaks into 

 fibres if it grows beyond its natural length. This is due to the 

 crumbling of the inter-tubular substance. 



In order that normal horn be secreted, it is essential that 

 the papillae and intermediate spaces in the coronary band, as 

 well as the sensitive laminae, be in a healthy condition ; for it 

 is found that the very tough and natural state of the wall is 

 maintained and preserved by the continual addition to it of 

 horny agglutinating cells, secreted by the sensitive laminae as 

 it (the wall) descends over the foot. 



Having seen, then, that the horn is secreted by the papiUae 

 and the inter-papillary substance, the reader can understand 

 that the brittle condition of the crust leading to sand-crack 

 depends upon a perverted condition of the secreting structures. 



Some horses are very liable to these cracks; and when a cure has 

 been apparently effected, they (the cracks) are very apt to return. 

 Such may be said to be naturally predisposed ; but the cause 

 must be looked for in the deteriorating effects of bad shoeing, 

 especially in that method which ignores the weight-bearing 

 properties of the sole. When the sole is weakened by the 

 drawing-knife, and shoes applied to the feet, more especially the 

 seated shoes which are put on the fore feet, the weight-bearing 

 surface is limited to the thickness of the wall ; and an amount 

 of weight is thus thrown on the crust calculated to overstrain 

 it, and to induce diseased action in the structures to which it is 

 attached, and by which it is secreted. Of itself horn is in- 

 capable of diseased action, being a secreted and not a formative 

 material. 



A sand-crack commences at the thin upper margin of the 



