DISEASES OF HORSES 283 



to which they are unaccustomed. Concussion from long drives on 

 dirt roads is at times productive of the same results, notably when 

 the weather is extremely warm, or at least when the relative change 

 of temperature is great. But the exhaustion of these circumstances 

 must prove an exciting cause as well as the long-continued concus- 

 sion. This combination of causes must also determine the disease 

 at times in hunters, for the weight of the rider increases the demands 

 made upon the function of these tissues, and their powers are the 

 sooner exhausted. 



(2) Overexertion, as heavy pulling or rapid work, even where 

 there is no immoderate concussion, occasionally results in this dis- 

 ease; but here also exhaustion is a conjunctive cause, for overexer- 

 tion can not be long continued without exhaustion. 



(3) Exhaustion is nearly as prolific a source of laminitis as is 

 concussion ; for when the physical strength is impaired, even though 

 temporarily, some part of the economy is rendered more vulnerable 

 to disease than otners. To this cause we must ascribe those cases 

 which follow a hard day's work, where at no time has there been 

 overexertion or immoderate concussion. 



Where one foot has been blistered, or where one limb is inca- 

 pacitated from any cause, the opposite member, doing double duty, 

 soon becomes exhausted, and congestion, followed by inflammation, 

 results. Where one foot only becomes laminitic, it is customary to 

 find the corresponding member participating at a later date ; not al- 

 ways because of sympathy, 'but because one foot had to do the work 

 of two. 



(4) Rapid changes of temperature act as an exciting cause of 

 laminitis by impairing the normal blood supply. This change of 

 temperature may be induced by drinking large quantities of cold 

 water while in an overheated condition. Here the internal heat is 

 rapidly reduced, the neighboring tissues and blood vessels con- 

 strained, and the blood supply of these organs greatly diminished, 

 while the quantity sent to the surface is correspondingly increased. 

 A cold wind or relatively cold air allowed to play upon the body 

 when heated and wet with sweat has virtually the same result, for 

 it arrests evaporation and rapidly cools the external surface, thereby 

 determining an excess of blood to such organs and tissues as are pro- 

 tected from this outside influence. 



A more limited local action of cold may excite this disease, by 

 driving through water or washing the feet and legs while the animal 

 is warm or just in from work. Here a very marked reaction takes 

 place in the surface tissues of the limbs, and passive congestion of 

 the foot results from an interference with the return flow of blood 

 which is being sent to these organs in excess. These are more apt 

 to be simple cases of congestion, soon to recover, yet they may be- 

 come true cases of laminitis. 



(5) Why it is that certain kinds of grain will cause laminitis 

 does not seem to be clearly understood. Certainly they possess no 

 specific action upon the lamina?, for all animals are not alike affected, 

 neither do they always produce these results in the same animal. 



