. Biology and man. Biology; Human beings. Association nerves Afferent nerve path. Effector (muscle) Efferent nerve path THE BURNT HAND DRAWS BACK When nerve endings in the skin are disturbed, an impulse travels up the afferent, or sensory, nerve cell. The disturbance is discharged to an efferent, or motor, nerve cell. Some is discharged also to an associated cell and transmitted to the brain. The stimulus in the motor nerve cell arouses contraction of muscle. The path from the receptor to the spinal cord to the effector is called a reflex arc vegetative. A person without a brain, or with one n
. Biology and man. Biology; Human beings. Association nerves Afferent nerve path. Effector (muscle) Efferent nerve path THE BURNT HAND DRAWS BACK When nerve endings in the skin are disturbed, an impulse travels up the afferent, or sensory, nerve cell. The disturbance is discharged to an efferent, or motor, nerve cell. Some is discharged also to an associated cell and transmitted to the brain. The stimulus in the motor nerve cell arouses contraction of muscle. The path from the receptor to the spinal cord to the effector is called a reflex arc vegetative. A person without a brain, or with one not working, is also largely vegetative, even if he sometimes uses his striped muscles vigorously. The Brain and Reflexes^ It is not diflftcult to show that animals— whether those with brains or those without—depend upon reflexes, or upon the reflex arcs in the nervous system. Let us suppose that a certain part of the sciatic nerve (the main nerve trunk running down the leg) were broken, destroying the continuity of the a^erefit fibers (see illustration, p. 284). One might then walk on carpet tacks or hot iron and not know it unless he happened to be watching his feet. Accordingly, one would not jump to avoid injury. Under these circumstances a person would still be able to move his legs or to jump if he wanted to. On the other hand, if the por- tion carrying e§erent fibers were cut, one would remain just as sensitive as ever to carpet tacks or hot iron or tickling; but he could not move his legs, no matter how much he wanted to. And they certainly would not move of themselves, for the part of the reflex arc connecting the spinal cord with the muscles would be broken. A large part of human activity may thus be seen to be mechanical, or ^See Nos. 4 and 5, p. 299. 282. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the origi
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