20 FIRST PRINCIPLES. 



The beneficial effect which oil has on the growth of hair, is 

 apparently due to the protection against moisture (including 

 perspiration) and against friction which it affords the hair. 

 Both in this case and in that of cold, the overgrowth is due 

 to waste being less than supply. The increased growth of the 

 skin and hair produced by a blister, appears to be caused 

 by an excessive supply of nutritive material (lymph), which 

 escapes through the walls of the small blood vessels during 

 the resulting congestion and inflammation. 



Pressure, such as that arising from saddle and harness gear, 

 causes epidermal thickening by increasing the supply of lymph 

 to the part. We see a similar process of epidermal growth 

 on the soles of the feet and on the palms of the hands of men 

 who respectively walk and row a great deal. In human 

 beings, congestion due to pressure when carried to excess by 

 the wearing of tight boots, produces corns. The more evenly 

 pressure is distributed, the less violent are its effects, as we 

 find in the difference between walking over gravel in thin 

 boots, to that in thick ones. As congestion in excess is liable 

 to become developed into inflammation, we should be careful 

 to distribute the pressure of harness and saddlery as evenly 

 as possible over the surface on which it falls. We are all 

 aware of the stimulating action which pressure has on the frog 

 of a horse's foot. I am indebted to Professor Halliburton for 

 the hint that epidermal overgrowth, like accumulation of fat, 

 is simply a case of supply being in excess of waste. 



Dr. W. H. Willcox has kindly pointed out to me that 

 various forms of disease are characterised by overgrowth of 

 epidermis. Thus, in human tuberculosis, the hair of the scalp 

 and the eye-lashes tend to grow long ; the skin gets coarse 

 and thick ; and the body often becomes covered with fine 

 downy hair. In cases of myxcedema with weak circulation, 

 and in those of congenital heart disease, the skin is thick and 

 the hair coarse. 



