Thumps Synchronous diaphragmatic flutter. The diaphragm contracts in the rhythym of the heartbeat. Usually due to an electrolyte disturbance, which presumably alters the membrane potential of the phrenic nerve, causing it to depolarize along with the electrical impulse which is propagated during cardiac muscle depolarization. In my experience, low calcium in distance horses is the most common presentation. In the above example, the cranial mediastinum is the portion nearer the head.
Thymus gland shrinks as the horse ages.
I think the chances of a wreck are far lower with the judicious use of a twitch than with a bunch of ropes and surcingles. As far as tying up a leg goes, I suspect that there is a fair chance of the horse falling down and damaging a knee compared to the same horse grimacing and rubbing his nose a bit after I remove the twitch. While I occasionally have need to resort to some of the more exotic methods of restraint previously posted, I find neuroleptoanalgesia and twitches to be the most useful in day to day practice. Often, the less restraint, the better. Tying Up Browse Dr. Newell's article on trail horse problems at her ftp server.