. Biology and human life. Biology. 2l8 BIOLOGY AND HUMAN LIFE. Fig. 109. Reflex arc Stimulation of the re- ceiving end a of an afferent nerve A leads to a discharge of en- ergy to all parts of the neuron, including the fine terminals, or dendrites, d. The dis- charge passes over to connected nerves, as the efferent nerve £, by way of the interlacing dendrites, or synapse, s: The discharge in E leads to stimulation of the organ with which it is connected, as a muscle M. Starting from a, in the spinal cord the disturbance is reflected by one of the side branches, or collat- erals, c, oi A, throu
. Biology and human life. Biology. 2l8 BIOLOGY AND HUMAN LIFE. Fig. 109. Reflex arc Stimulation of the re- ceiving end a of an afferent nerve A leads to a discharge of en- ergy to all parts of the neuron, including the fine terminals, or dendrites, d. The dis- charge passes over to connected nerves, as the efferent nerve £, by way of the interlacing dendrites, or synapse, s: The discharge in E leads to stimulation of the organ with which it is connected, as a muscle M. Starting from a, in the spinal cord the disturbance is reflected by one of the side branches, or collat- erals, c, oi A, through the synapse 5 into E, leading to a movement by the contraction of M working. A lowering of the amount of sugar in the blood starts the liver discharging more glycogen. The sight of a ghost increases the flow of sweat from the skin glands. 3. Nerve. Sometimes a stimulus fails to bring about an immediate reaction, but pro- duces instead some change in the nerve cells of the brain. You hear a word, one of many- used in sentences—a lecture, a scolding, the rules of a game. You do nothing about it, apparently, at the time, but later you give evidence that the word had an effect: you recall the word when needed, you do what you were told to, and so on. The whole behavior of a man or an ameba could be described as a system of reactions to stimuli. We cannot always recognize the stimulus; we cannot always discover the connection between the stimulus and the re- action ; nevertheless both the single cell of the ameba and the nervous system of man, made up of many millions of cells, can best be understood in this way. 179. Reflexes. When you are tickled, you draw away the touched part. When some- thing gets close to your eye, you wink. When the illumination is suddenly increased, the pupil of your eye contracts; when it is di- minished, the pupil expands. When some- thing tickles the inside of your nose, you sneeze. When a solid particle touches the lining of your windpipe, you cou
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