Comparison between the winter and summer coats of horses and other animals, proves that cold, which reduces the supply of blood in the surface of the body, has a stimulating effect on the growth of the epidermis. In seeking for an explanation of this apparent anomaly, we must examine the manner in which cold affects the respective processes of supply and removal of this structure. Epidermis receives its nourishment from lymph (fluid portion of the blood) supplied by the blood vessels of the true skin. As it possesses no blood vessels, its removal cannot be effected by means of the blood; but is accomplished by exfoliation, and by contact with external objects, as we may see in the flaking off and wearing down of the hoofs of an unshod horse. The exfoliation of hair is accomplished naturally by the shedding of the scales which cover the hairs, as we may observe by the thinned-out condition of their ends. The exfoliation of epidermis is hastened by the action of perspiration. Thus, cuticle can be far more easily rubbed off the skin of a man who is sweating freely, than off that of a man whose body is cool. Also, vigorous grooming tends to make the coat both fine and short. Contrary to the general opinion that brushing stimulates the growth of hair, we have the fact that the Sikhs, who have longer hair than any other people in the world, keep their hair as a rule closely coiled up on their heads, and undo it, for purposes of cleaning, only after long intervals of time. I may explain that their religion forbids them to cut their hair. When stationed in the Punjab, which is the country of the Sikhs, I have seen many of them making their, probably, annual capillary toilette, and have often noticed among them, instances of the hair reaching to the ground, while the owners were standing upright. In fact, it is no uncommon occurrence to find Sikhs with hair 6 ft. long ! I think we may safely attribute the extraordinary length of their hair to comparative absence of friction, and to the protection against the action of perspiration afforded by the compact condition which the hair is made to assume on the head.

The horns of animals which wear this kind of epidermal growth, are almost always more massive in cold countries than in hot ones, among members of the same species, as we see in reindeer, ovis ammon, and ibex. Also, observations in the respective countries will show us, that the hoofs of Tibet and Finnish ponies are thicker than those of Indian horses. We must here bear in mind that the colder a climate, the dryer it is, other things being equal.

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 myxoedema with weak circulation, and in those of congenital heart disease, the skin is thick and the hair coarse.