This term is used to indicate certain elevations of the surface of the skin, the centre of which presents a pale colour, having the appearance produced by the sting of the nettle. Wheals are caused by sudden dilatation of the blood-vessels, followed by the escape of fluid from them into the centre of the swelling. These appearances, however, are not generally recognizable in the horse, owing to the colour of the skin and the covering of hair.
Papules, meaning pimples, are of various kinds, ranging from minute specks due to excess of blood in the papillae of the skin, to the larger elevations caused by distension of the follicles, or by the deposition of lymph around them, or by solid deposits in the true skin. There are also papules or pimples formed by the collection of sebaceous secretion within the follicles, and others resulting from hypertrophy or overgrowth of normal structures, forming minute fibrous excrescences or small warts.
Vesicles are really small bladders or blisters containing a watery fluid; they naturally vary in size according to the amount of fluid which they contain. When the bladders are sufficiently extensive to be described as large bladders they are termed bulla; thus, as the result of an ordinary blister, large and small water bladders, otherwise vesicles and bullae, will easily be distinguished.