VASO-MOTOR CENTRE. 633 



had been in the habit of doing, only under the direction of the 

 higher centre. From this we conclude that, though the cord con- 

 tains local vaso-raotor centres distributed throughout its gray 

 matter, these are all under the direction and control of the vaso- 

 motor centre in the medulla, and this centre is really the chief 

 station from which impulses destined to affect the whole organism 

 must emanate. 



This arrangement is quite comparable with that by which the 

 ordinary muscles are made to contract. When the will causes a 

 muscular contraction, the impulse starting from the cerebral cor- 

 tex does not travel directly to the muscle, but it passes from the 

 brain to certain cells in the cord and thence to the muscles. In 

 fact, to these spinal agents the ultimate arrangement and coordi- 

 nation of the act is confided. So, also, the chief vaso-motor cen- 

 tre in the medulla executes its orders through the medium of 

 numerous under centres placed at various stations along the cord. 



The vaso-motor centres like nearly all other controlling 

 groups of ganglion cells must be considered to be made up of 

 two parts antagonistic one to the other, viz., a constricting and a 

 dilating centre, the impulses from which commonly travel along 

 separate nerve channels. The constricting impulses are mainly 

 distributed by the sympathetic nerve, while the dilating impulses 

 generally run in the ordinary peripheral nerves, which are em- 

 ployed in calling forth the ordinary function of the part in ques- 

 tion. This is chiefly true of the internal organs, but in the limbs 

 all the nerve channels are commonly collected together to form a 

 single nerve. 



From what has been said as to the wide distribution of centres 

 influencing the bloodvessels, an attempt to localize exactly the 

 position of the medullary vaso-motor cells is not satisfactory. In 

 the lower animals frogs the cells are evenly diffused through- 

 out medulla and cord. In man the localization is difficult to 

 demonstrate, though we have reasons for thinking it much more 

 definitely circumscribed than in the lower animals. In the rab- 

 bit it has been tolerably accurately localized to the floor of the 

 fourth ventricle, in the immediate neighborhood of the respiratory 



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