Owing to the hair and the colour of the skin the blood can rarely be seen in a horse's skin the same as it can in a man's; but it is there, flushing the parts now more, now less, depending on the temperature and other local conditions. It is only in the pink skin of horses with white heels that the blood vessels can be seen, giving the part, especially under the influence of exercise, the bright, ruddy appearance seen in the active skin of a man at work. The vessels in the skin which carry the blood are hair like in diameter and innumerable. Their structure is so delicate that very little pressure is sufficient to keep the parts empty, as can be easily demonstrated on one's hand; the white mark which follows temporary pressure on the back of the hand is due to the minute blood vessels being emptied of their blood; as soon as the blood once more enters the white mark disappears.

Friction may cause galls
The outer or scaly covering of the skin is mainly for the purpose of protection, it protects the delicate parts beneath, and as it gets worn away by friction is replaced from below. Ordinarily the production of scales is equal to the wear and tear, but when friction causes wear and tear to predominate over production, injury arises and the sensitive parts beneath are then exposed. A rub to a back, a gall to a shoulder, are illustrations of excess of wear and tear over production, and represent the simplest form of gall.

Long backs
Long backs are a source of weakness, and as a rule they are poorly developed, while their length is in itself a weakness; such backs are frequently associated with long narrow loins, one of the greatest faults a troop horse can possess. The loins can hardly be too short and can never be too wide.

Shape of backs classified
Backs vary so much in shape as any other part of the body; if careful measurements are made it is surprising how few have the same shape and size of back, yet a broad classification of backs may be made; here, for example, is a table in which the extremes are shown:

High withers        Low withers
Short backs         Long backs
Hollow backs        Wide backs
Narrow backs

And between these extremes any degree may be met.