22 FIRST PRINCIPLES. 



rays of a tropical sun. In this respect, it is much more 

 effective than pith sun-protectors placed over the top of the 

 head, poll, and loins. We should bear in mind that the con- 

 nection between the brain and the eyes by the optic nerve and 

 its expansion, the retina, is short and direct. Most persons who 

 have lived in the tropics must have felt the cooling effect on 

 the entire system produced by the wearing of blue, green, or 

 neutral-tinted glasses in the open on a very hot sunshiny day. 

 As evaporation implies loss of fluid, a full supply of drinking 

 water is an effective means for cooling the body of an animal 

 exposed to great heat. 



Exercise raises the temperature by increasing tissue change, 

 as we learn from the fact that it causes the lungs to give off 

 much more carbonic acid than they do during rest. At the 

 same time, it stimulates the evaporation of moisture from the 

 lungs and skin ; and the nerves preserve the balance of healthy 

 temperature. 



When the influence of any cooling process, such as that of 

 external cold, is greater than the influence of normal heat 

 production, there will be undue bodily waste owing to 

 increased chemical decomposition being necessary to obtain 

 the extra heat. Consequently, when horses are " turned out " 

 in cold weather, they will require more food to keep them in 

 " condition " than if they were comfortably housed. 



REACTION AND CHILL. 



The term reaction, when applied to conditions of health, 

 signifies the more or less rapid readjustment of a functional 

 balance which has been recently disturbed, as for instance, by 

 going out for a few moments into sharp, frosty air, or by 

 taking a plunge into cold water. If the action of cold on the 

 skin in either of these cases produces a healthy glow over, 

 respectively, the surface of the face or that of the body, 

 we have healthy reaction, which is often accompanied by 



