External. Internal. Traumas, abrasions, excoriations, lacerations,
contusions, compression, radiating heat, scalding, incandescent
objects, solar heat, chemical caustics, cold, freezing, kicks, bites,
tusk horn or claw wounds, stings, venoms, envenomed bites, road dust,
sweat, excretions, sebum, mineral and vegetable poisons, essential
oils, fungi. Hyperæmia, exudation, depilation, sudation, moulting,
climatic changes, unwholesome or irritating foods, ptomaines,
alimentary fermentations, hepatic, renal or blood disorders, altered
innervation, youth, age, temperament, heredity. Experimental nervous
cases.
These may be =external= or =internal= or both.
=External Causes.= Some affections of the skin are due to external
causes exclusively, while in others the local cause of irritation is
accessory but no less important in maintaining the trouble. Among the
more prominent external factors may be named: traumatisms, abrasions,
excoriations, lacerations, contusions, compression, radiating heat,
boiling water, hot or incandescent solids or liquids, solar heat,
chemical caustics and irritants, cold, freezing, injuries by harness,
kicks, lacerations with teeth, tusks, horns or claws, stings, bites,
(leeches, snakes, etc.), venoms, (snake, toad, etc.), road dust and
sweat, liquid fæces or urine, excess of sebum in sheath or vulva,
mineral poisons (mercurial, iodides, bromides, arseniates, caustic
alkalies, caustic salts, etc.), vegetable poisons (croton, bryonia alba
and dioica, heracleum or cow parsnip, polygala or milk wort, cyclamen or
sow bread, polygonum hydropiper, mustard, œnanthe, cicuta, hypericum
perforatum and androsaemum, rhus toxicodendron, radicans and venenata,
capsicum, pepper, radish, Indian syringa, anemone nemorosa and patens,
ranunculus acris, scelerata, flammula, mericatus and bulbosus, cytisus,
euphorbium and the essential oils of turpentine, origanum, lavender,
etc.), fungi of musty food, ergot, etc.
=Internal Causes.= Among these are all conditions that induce stasis in
the capillaries or lymph vessels, active hyperæmia, exudation,
depilation, profuse perspiration, shedding the coat, exposure to cold,
chill, etc., sudden access of warm weather, poor and insufficient or
rich, stimulating food, cotton seed meal, indian corn, buckwheat, purple
clover, animal food (in dogs), spiced food, food spoilt by wet and
cryptogams, indigestions, gastric and intestinal fermentations, hepatic
disorders, renal disorders with imperfect elimination, blood disorders,
and nervous disorders which entail vaso-motor changes. Early age
predisposes to some affections (variola, warts); old age to others
(eczema). A nervous temperament in horses favors the drier eruptions
(pityriasis), a lymphatic temperament the exudative (grease, canker,
moist eczema). A hot, moist season favors most skin affections (eczema,
acariasis, etc.), dry insolation others (erythema) and cold still others
(chillblains, frost bite, chaps, etc.). Some eruptions are at first
summer troubles, disappearing on the advent of cold weather, yet in time
the predisposition increases, or the inflamed skin becomes less
resistant and the disease becomes permanent. A marked predisposition in
certain animals, appears to inhere in the constitution and proves
hereditary in the family (Blain, Lafosse, Cadeac). Nervous causes have
not been satisfactorily traced in the lower animals, yet the dermatitis
of the face and neck caused by the experimental lesion of the cervical
sympathetic or its connecting ganglia shows clearly enough how any
portion must be affected by disorder of its trophic or vaso-motor
nerves. Charcot found that experimental lesions causing inflammation of
the spinal cord, led to trophic changes and finally gangrene of a
corresponding part of the skin. Babesiu and Israï injected oil of
mustard in one side of the cord, in three dogs, and found in several
days a vesicular eruption and atrophy of the skin on the same side, a
result they supposed of the myelitis.