Abstract
The strength of contraction of the rectus abdominus muscle of the frog immersed for varying periods (15-60 seconds) in weak acetylcholine solution (about 1 part in 500,000 parts of Ringer's solution, depending on the sensitivity of the preparation) was found to increase with the hydrogen ion concentration of the environment. Such effects occurred within a range of pH 7.5 to pH 5.0. Lactic, phosphoric, and carbonic acids produced comparable effects. These results agree with those previously reported in the rectus abdominus. 1 , 2 However, when the sartorius muscle was used it was found to respond with a weaker contraction under increasing hydrogen ion concentration of the environment. This unexpected finding led us to make comparative observations on a number of muscles of the frog, turtle and alligator. The rectus abdominus, mylohyoid and geniohyoid of the frog, mylohyoid of the turtle, and diaphragm of the alligator, muscles which contribute to the respiratory act, were found to show a progressively increasing strength of contraction with an increasing cH of the acetylcholine-containing environment, while the sartorius, peroneus longus andgracilis minor of the frog, muscles possessing primarily a locomotor function, showed either a consistent depression or an initial augmentation followed by an early depression.
This differential effect of acid on these two groups of muscles is of interest in suggesting, for the lower forms at least, that the control of breathing is not strictly a central reflex phenomenon but is supported by a peripheral motor adjustment as well. The chemical (acid) control of respiration may thus be extended to the respiratory muscles as well as 5 to the carotid body and the intra-cranial portions of chemically sensitive control centers.
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