The diaphragm is a muscle which, with its broad central tendon, separates the chest from the abdominal cavity. Under some circumstances it is excited to violent contraction, which is repeated at shorter or longer intervals, and sometimes continues for several hours. The contractions are sudden and spasmodic, and so forcible as to shake the entire body from end to end. The morbid action is accompanied by a thumping noise, which might be mistaken for palpitation of the heart, were it not that the sound emanates from a more backward position in the animal's body, and the flanks are considerably agitated at the same time. Moreover, the contractions of the diaphragm and the stroke of the pulse do not occur simultaneously. The heart, however, is more or less sympathetically affected, but in no case should there be any difficulty in distinguishing between derangement of the one organ and of the other, and especially if the moment of the heart's beat be carefully noted and compared with that of the contraction of the diaphragm.
The cause of this morbid action is not very clearly understood. It is known to have followed severe exertion, as after a good burst of speed in the hunting-field, but the writer has known it to occur in the stable after days of idleness, and on one occasion to usher in an attack of colic.
The suddenness of its onset and disappearance, and its behaviour while present, seem to mark it out as a neurotic affection, in which the pneumo-gastric nerve is most likely specially concerned.
Diffusible stimulants with anodynes, enjoined with warm clothing to the body, are measures most likely to subdue the morbid activity of the muscle. These should be followed by a dose of aperient medicine and a short period of careful dieting.