SECTION III 



THE REGULATION OF THE RESPIRATORY 



MOVEMENTS 



EACH movement of inspiration involves the co-ordinated activity 

 of a large number of muscles. Thus the diaphragm and the inter- 

 costal muscles must come into action at the same time, and the 

 extent to which they contract will determine the depth of the inspira- 

 tion. Similarly, they must cease to act simultaneously if the act of 

 expiration is to take place. The rhythm and extent of the alternate 

 contractions and relaxations of the respiratory muscles are determined, 

 as we have seen, by the needs of the organism as a whole. These 

 respiratory movements are regulated so that the total ventilation of 

 the alveoli shall be sufficient to meet the gaseous exchanges of the 

 body. Whether the organism consumes 250 or 1000 c.c. of oxygen 

 per minute, the respiratory movements keep the composition of the 

 gas in the alveoli at a practically constant level. 



The muscles involved both in inspiration and expiration can only 

 be thrown into activity by the intermediation of nerves. Each act 

 of inspiration involves a discharge along a number of nerves, e.g. 

 the facial to the muscles moving the alas nasi, the vagus to the muscles 

 of the larynx, the branches of the cervicle and brachial nerves to the 

 muscles of the neck, the phrenic nerves to the diaphragm, and the dorsal 

 nerves to the intercostal muscles. The fibres making up these nerves 

 are derived from nerve-cells of the anterior horn, situated at various 

 levels in the medulla and spinal cord. In each act of inspiration or 

 expiration the activities of all these groups of cells must be brought into 

 relation among themselves, as well as with the needs of the organism 

 for oxygen and for the elimination of carbon dioxide. It is conceivable 

 that the co-ordination of the activities of the various motor nuclei 

 might be attained by the provision of communicating nerve-paths 

 joining the centres among themselves, and by a sensibility of all these 

 centres to the gaseous contents of the blood as well as to the influence 

 of afferent impressions from the periphery. A much more efficient 

 co-ordination, however, would be effected by the subjection of these 

 motor nuclei to the action of some specialised portion of the central 

 nervous system which would act as a receiving centre for afferent 

 impressions from the lungs and surface of the body, and would be 



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