CHAPTER VI 

 BRAIN AND NERVE 



THE next thing we must consider is the means whereby the 

 various activities of the animal body are linked together and 

 co-ordinated. What has already been said indicated the existence 

 of such co-ordinations, but it will be useful to give an illustration. 



A man. then, engages in sudden vigorous exercise. First of 

 all he flushes and feels warm (though the temperature of his 

 body does not increase appreciably). He breathes more deeply 

 and rapidly than is normally the case. His respiratory move- 

 ments also deepen and quicken. The rate of the heart-beat is 

 increased. The skin becomes moist. 



Apart altogether from the series of nervous impulses that issue 

 from the spinal cord and brain to actuate the muscles that are in 

 movement, and apart also from the series of impulses that stream 

 into the central nervous system from the muscles and skin, there 

 are other mechanisms in operation. These effect regulations of 

 functioning. First of all the mechanical work done by the 

 contracting muscles leads to a production of heat and carbonic 

 acid, both being due to the oxidation of chemical substances 

 contained in the muscle fibres. The carbonic acid is absorbed 

 by the blood-stream, so that the percentage of this substance in 

 solution tends to rise. Now there is a little group of nerve cells 

 (the respiratory centre or " vital knot ") in the medulla, and this 

 is susceptible to changes in the quantity of oxygen and carbonic 

 acid carried in the blood that traverses it. When this per- 

 centage of C0 2 is greater than normal, the automatic activity of 

 the centre is accelerated, and vice versa. Rhythmic impulses 

 issue from it at an average rate, and actuate the muscles of the 

 chest wall and diaphragm. Therefore, when the quantity of C0 2 

 in the blood increases as the result of increased muscular activity, 

 the respiratory movements are automatically accelerated; there 

 is a greater ventilation of the lung cavities, more oxygen is 

 absorbed and more C0 2 is excreted, and the composition of the 

 blood tends to go back to that which is normal. 



The heart-beat is a rhythmic automatic one, and it occurs at 



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