FUNCTIONS OF THE MEDULLA OBLONGATA. 515 



whereby the impulses producing the respiratory move- 

 ments are reflected. 



The power by which the medulla oblongata governs and 

 combines the action of various muscles for the respiratory 

 movements, is an instance of the power of reflexion, which 

 it possesses in common with all nervous centres. Its 

 general mode of action, as well as the degrees to which 

 the mind may take part in respiration, and the number 

 of nerves and muscles which, under the governance of the 

 medulla oblongata, may be combined in the forcible respi- 

 Tatory movements, have been already briefly described (see 

 p. 225 et seg.). That which seems most peculiar in this 

 centre of respiratory action is its wide range of connection, 

 the number of nerves by which the centripetal impression 

 to excite motion may be conducted, and the number and 

 distance of those through which the motor impulse may be 

 directed. The principal centripetal nerves engaged in 

 respiration are the pneumogastric, whose branches supply- 

 ing the lungs appear to convey the most acute impression 

 of the "necessity of breathing." When they are both 

 divided, the respiration becomes slower (J. Reid), as if the 

 necessity were less acutely felt : but it does not cease, and 

 therefore other nerves besides them must have the power 

 of conducting the like impression. The experiments of 

 Volkmann make it probable that all centripetal nerves 

 possess it in some degree, and that the existence of imper- 

 fectly aerated blood in contact with any of them acts as a 

 stimulus, which, being conveyed to the medulla oblongata, 

 is reflected to the nerves of the respiratory muscles : so 

 that respiratory movements do not wholly cease so long as 

 any centripetal nerves, and any nerve supplying muscles 

 of respiration, are both in continuous connection with the 

 respiratory centre of the medulla oblongata. The circulation 

 of imperfectly aerated blood in the medulla oblongata itself 

 may also act as a stimulus, and react through this nerve- 

 centre on the nerves which supply the inspiratory muscles, 



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