On reciprocal innervation of antagonistic muscles.─ Tenth note

I. In previous notes on this subject the examples of reciprocal innervation taken from the limb muscles have usually been from muscles acting at the knee. An advantage offered there is that the vasti and crureus , extensors of the knee, act directly on that one joint alone. A muscle acting on two joints is often an extensor of one of the joints and a flexor of the others. When such a muscle is thrown into contraction by reflex action, there may be a dilemma as to whether it is as an extensor or as a flextor, that it is acting, and the answer will depend partly on the kind is gastrocnemius ; it extends the ankle and flexes the knee. It is, therefore, of interest to examine how this muscle behaves when its antagonist, tibialis anticus , a flexor of the ankle, is thrown into contraction by reflex action. Stimulation of any among various points of skin, or of the proximal end of various afferent nerves of the leg, excites reflex contraction of tibialis anticus. A nerve often used for the purpose in my observation has been the cutaneous branch of the peroneal nerve about a centimetre above the ankle (cat). In the reflex which then occurs in the limb, the following points can be made out. The ankle is flexed, and with the flexion of ankle occurs also flexion at knee and hip. If the anterior crural, obturator, hamstring, and popliteal nerve trunks, the insertions of iliopsoas, and psoas parvus, and the origin of pectineus muscles be severed, active flexion of knee and hip are excluded from the reflex. The active flexion at ankle still persists. On inspecting the tendons of the pretibial flexors of the ankle, that of tibialis anticus is seen to be drawn upward at each reflex, and with it also those of extensor longus digitorum and peroneus longus, but in my experience these latter are retracted less. If, in addition to the nerves and muscles mentioned above as severed, but the branch from the popliteal nerve to the gastrocnemius be spared, and that muscle be exposed to inspection, usually no change in that muscle appears.