It is well known that various forms of skin eruptions are due to the attacks of various insects, as lice, fleas, gnats, gadflies, etc, also from the ravages of acari; and certain vegetable parasites which establish themselves in or on the integument, for example, the well-known ringworm fungus. Eruptions also occur as the consequence of the contact of such agents as mustard, paraffin, solution of carbolic acid, arsenic, and even soft soap; when this agent has been used in strong solution for the purpose of washing the skin, and a quantity of it has been carelessly left upon the upper part of the body, extensive eruption along the centre of the back has been observed, in cattle particularly. It will be noticed that the degree of irritation produced is largely dependent upon the sensibility of the skin; when this is thin and delicate, the application of mustard or ammonia will often produce extensive erythema, followed by the formation of vesicles, and ultimately desquamation of the cuticle, and maybe a superficial ulceration, while the same agent applied to an animal with a coarse skin produces little or no effect. The use of arsenic as a caustic in the treatment of warts has sometimes caused extensive inflammation of the skin surrounding the wart, with considerable slough-ing. It has already been stated that horses working where the ground is covered with lime suffer from inflammation of the skin of the legs, accompanied by cracks or fissures. Sometimes inflammation of the glandular structures of the skin arises out of the same cause, accompanied by a considerable discharge of sebaceous fluid.

Internal agents, such as articles of food and various drugs, give rise to eruptions in the skin of susceptible subjects; the eruption known as nettle-rash following the eating of mussels and other shell-fish by man is an instance of an eruption following the consumption of certain kinds of food. Medicinal agents are responsible for a large number of disorders of the skin, of the erythematous, papular, vesicular, and bullous type, all of which are known as drug eruptions. The occurrence of these diseases in the skin of the lower animals from the use of different drugs has not been recognized by veterinary writers, but there is no reason to doubt that they may occur from the continued employment for medicinal purposes of bromides, iodides, mercury, arsenic, salicylic acid, and other agents. Salicylic acid certainly has a marked local effect when given in considerable doses to cattle for the removal of large pendulous warts; after the use of the agent for this purpose for some weeks, the skin near the warts becomes inflamed, and the warts fall off in succession, leaving the raw surface, which rapidly heals. The use of the agent is then, of course, discontinued.

The fact of the occurrence of eruptions of different kinds from the action of external and internal agents should, under all circumstances, be taken into account in forming a diagnosis.