General Causes of Skin Diseases. 459 



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, con- 

 tusions, compression, radiating heat, boiling water, hot or in- 

 candescent 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 

 faeces 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, cycla- 

 men or sow bread, polygonum hydropiper, mustard, cenanthe, 

 cicuta, hypericum perforatum and androsaemum, rhus toxi- 

 codendron, radicans and venenata, capsicum, pepper, radish, 

 Indian syringa, anemone nemorosa and patens, ranunculus acris 

 scelerata, fiammula, mericatus and bulbosus, cytisus, euphor- 

 bium 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 hypersemia, exu- 

 dation, depilation, profuse perspiration, shedding the coat, ex- 

 posure to cold, chill, etc., sudden access of warm weather, poor 

 and insufficient or rich, stimulating food, cotton seed meal, In- 

 dian corn, buckwheat, purple clover, animal food (in dogs), 

 spiced food, food spoilt by wet and cryptogams, indigestions, 

 gastric and intestinal fermentations, hepatic disorders, renal dis- 

 orders with imperfect elimination, blood disorders, and nervous 

 disorders which entail vaso-motor changes. Early age predis- . 

 poses 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 (chillblaines, frostbite, 

 chaps, etc.). Some eruptions are at first summer troubles, dis- 

 appearing on the advent of cold weather, yet in time the predis- 

 position increases, or the inflamed skin becomes less resistant 



