Human physiology / by John Thornton.

  • Thornton, John
Date:
1899
Catalogue details

Licence: Public Domain Mark

Credit: Human physiology / by John Thornton. Source: Wellcome Collection.

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    the transference of an afferent impulse along the optic nerve to a centre in the brain, and an efferent impulse is then sent along the third cranial nerve to the circular fibres of the iris, and their contraction follows without our knowledge. Sneezing, on the other hand, is a reflex action set up by irritation of the nasal branch of the fifth nerve conveying impulses to the respiratory centre in the medulla that lead to the action of expiratory muscles, and of this we are usually conscious at the time or may become conscious subsequently. F'inally, we may note that reflex actions may often be inhibited or arrested by an effort of the brain (will), though in some cases the stimulus may be so powerful as to defy inhibition. 45. Voluntary Actions in contrast with Reflex Actions.— A large class of conscious efferent impulses appear to arise in the brain without any immediate afferent impulses. They proceed from the will or choice of the individual. These volitional or voluntary impulses differ from the immediate motor responses to stimuli that constitute reflex acts by seeming to be free, irregular, spontaneous, or even chosen in spite of exciting impressions. There is no doubt some preceding stimulus, but between the sensation and the motion the brain compares and judges. Drawing a plan, writing a theme, or solving a problem involve conscious voluntary acts. But it is easy to see that there is difficulty in fixing the boundary between voluntary and reflex acts, and that acts that are reflex ordinarily may be modified by the influence of the will. 46. Velocity of Nerve Current.—The rate ot transmission of nerve impulses along a nerve-fibre has been found to be about 100 feet per second (light travels 186,000 miles per second, and electricity 200,000 miles per second). This rate has been directly measured by stimulating one of the nerves for the small muscles of the hand near its origin in the neck and at another point in the forearm (the nerve coming near the surface at these places), and observing by the help of electric signals the difference of time between the contraction of the muscles when the nerve is stimu- lated at the distant position and the near position. Knowing the rate of con- duction in nerve-fibres, it is possible to determine the time occupied by a nerve centre in its work. This is done by stimulating a sensory or afferent fibre of the spinal cord, and observing the length of time between throw- ing in a shock and the commencement of the muscular contraction on the motor or efferent side. Taking from this whole time the time required to pass along the known length of nerve-fibres and the time lost in the muscular nerve-ending, the time occupied in the nerve centre in the reflex act is
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