Abstract
Tonic oscillations of the auricular musculature in certain species of terrapins is a well established fact. 1 This tonicity apparently depends upon the presence of auricular smooth muscle 2 since tonus is not found in the auricles of species which do not contain it. Tone is increased by pituitrin, histamine, nicotine, potassium chloride and barium chloride and depressed or abolished by epinephrine and tyramine. 4 , 5 Tonus is generally believed to be increased by stimulation of the vagus. 1 , 6 Recently, however, it has been reported that the tonus mechanism is not innervated by parasympathetic motor nerves since it is not influenced by physostigmine or atropine. 7
Therefore, for a more complete understanding of this mechanism, it seemed necessary to know the action of acetylcholine on tonus.
In the experiments here described, the terrapin (Pseudemys elegans) was used. The auricles were carefully cut away from the ventricle and large veins and suspended by threads between the tip of a glass L-tube and a heart lever. This was then suspended in a muscle bath and bathed with Ringer's solution. A small stream of air was delivered to the solution by the L-tube throughout the experiment. The solutions used were at room temperature (18-20°C.). The concentrations of the solutions referred to in the text indicate the concentration of the drug in the muscle bath. In Ringer's solution, strong rhythmic contractions were obtained, these usually continuing for many hours. Only occasionally were tonus waves absent.
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