Abstract
In previous communications 1 we reported the results of tonus measurements carried out on the quadriceps femoris and triceps brachii muscles under normal innervation and following elimination of the sympathetic, in case of the quadriceps femoris, and either the sympathetic or the somatic nerve supply, in case of the triceps brachii muscles. The results of these measurements indicate that an important component of the tonus of a resting muscle, as manifested by the resistance it offers to passive extension, is mediated through its sympathetic innervation.
The majority of our tonus measurements, the results of which were previously reported, were carried out on cats and dogs under light ether anesthesia. In some instances measurements carried out in this manner were compared with measurements carried out on the same animal while in the waking state. In all these instances the tonus curves obtained while the animal was under light ether anesthesia were essentially similar to those obtained while the animal was in the waking state.
Nevertheless, it has been objected that even light ether anesthesia may vitiate the results of tonus measurements. In order to obviate this objection, and to secure additional checks on our previous experiments, tonus measurements were carried out on a goodly number of animals (cats and dogs) following section of the brain stem just above the midbrain. As observed by Magnus (1916), 2 the tonus of skeletal muscles is not appreciably altered by section of the brain stem at this level. Animals successfully operated in this manner exhibit no voluntary movements, but their spinal and medullary reflex mechanisms remain intact. Spinal and medullary reflexes, especially those involved in progression, are readily elicited, in such animals, as soon as the immediate shock of the operation has subsided.
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