Lesions of various kinds, such as shoulder bruises, abscesses, abrasions and callouses, saddle bruises, callouses, abscesses or fistulæ cause acute pain whenever the effort is made, and render the animal more impatient and indisposed to try again. Too small a collar or one that fits badly (too narrow, uneven) has often a similar effect. Among other causes may be named a hard bit harshly used, a sharp edge of the lower jaw bone where the bit rests in the interdental space, sores of the buccal mucous membrane in this situation, and caries or necrosis of the superficial layer of the bone. Also chaps, ulcers, or cancroid of the angle of the mouth. Young horses, that are as yet imperfectly trained, are more readily driven to balk than old trained animals. Mares are more subject to the vice than geldings, by reason apparently of a more nervous disposition, but much more because of the excitement to which they are subjected, under the periodic returns of _heat_. Pench speaks of rare hereditary cases in which the habit is uncontrollable and the animal incurable. Friedberger and Fröhner accuse chestnut and sorrel horses as being especially liable to balk. However started the continued exercise of the act fixes it as an incurable habit a virtual psychosis. Yet the inclination of the animal, his likes and dislikes to a certain extent control its manifestations, thus a horse rarely balks in going home, and shows it mostly in going in the opposite direction, and above all on a new or unknown road. The _Symptoms_ vary greatly in different cases. One animal stands stock still propping his legs outward and absolutely refusing to budge. This may occur even in the stall when it is attempted to take the animal out. When on the road he is usually willing to turn and go back, but no persuasion by voice or whip can force him forward. Other horses make ineffective plunges forward but never throw weight enough into the collar to overcome any resistance. Still others stamp, bite, throw themselves to one side rather than forward, rear up, strike with the fore feet, and if whipped kick with the hind. Some will throw themselves down and struggle in this condition. Under the saddle the animal may crowd against a wall, rear, kick, buck or even throw himself down in his efforts to dislodge the rider. These violent manifestations however rather belong to vice than mere balking. Trembling, perspiration, frequent rejection of urine, and general acceleration of pulse and breathing may manifest a severe nervous disorder. _Diagnosis._ It is often important to pronounce upon the exact nature of this trouble so as to determine whether the seller is responsible for a breach of warranty given or implied. As regards implied warranty a _sound price_ for an animal sold to do a given kind of work implies a mutual understanding that the animal is not physically or psychically incapacitated for such work.