. Physiology, experimental and descriptive . REFLEX ACTION. 39 Such actions as the struggling of the chicken, whose headhas been cut off, and of the brainless frog in drawing up thefoot when the toes are pinched, are examples of Reflex Actionof the Spinal Cord. When the toes are pinched, the nerve current, or nerveimpulse, runs up the nerve to the spinal cord. The gray mat-ter in the central part of the spinal cord receives the message,and sends back a nerve impulse to the muscles to make themshorten, and pull the foot away. Dorsal RootAfferent Sensor Fiber. Motor Fiber v Efferent w . „ . Vent
. Physiology, experimental and descriptive . REFLEX ACTION. 39 Such actions as the struggling of the chicken, whose headhas been cut off, and of the brainless frog in drawing up thefoot when the toes are pinched, are examples of Reflex Actionof the Spinal Cord. When the toes are pinched, the nerve current, or nerveimpulse, runs up the nerve to the spinal cord. The gray mat-ter in the central part of the spinal cord receives the message,and sends back a nerve impulse to the muscles to make themshorten, and pull the foot away. Dorsal RootAfferent Sensor Fiber. Motor Fiber v Efferent w . „ . Ventral Root. Fig. 15. Diagram of Reflex Action of the Spinal Cord. (After Landois & Stirling.) The sciatic nerve is composed of many fibers. If this nerveis traced outward, it is found to be continually subdividing,and sending small branches to the muscles, and finally in themuscles one fine nerve fiber goes to each muscle fiber. Manyfibers go on past the muscles to the skin. We can feel in anypart of the skin, and we can tell just where we are fibers from the skin, then, carry nerve impulses inward,as those going to the muscles carry impulses outward. In skinning an animal we see many bands of white ortransparent tissues connecting the skin and the sheaths of 40 NERVE BOOTS. the underlying muscles. This is connective tissue, as is thesheath of the muscle. We also see blood tubes running out tothe skin. And if we were to look very closely, we might seefine nerve fibers passing from the skin to join the nerve trunkin the legs and bo
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