VASOMOTOR CENTRE. 633 



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 

 cortex 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 

 coordination of the act is confided. So, also, the chief vasomotor 

 centre in the medulla executes its orders through the medium of 

 numerous under centres placed at various stations along the cord. 



The vasomotor 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 employed in 

 calling forth the ordinary function of the part in question. 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 blood vessels, an attempt to localize exactly the 

 position of the medullary vasomotor 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 rabbit 

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

 ventricle, in the immediate neighborhood of the respiratory and 

 cardiac centres. From this the nerves pass into the cord to the 

 spinal roots, by which they reach the sympathetic. 



The vasomotor centre exerts a tonic or continuing action on 

 the vessels, holding them in a state of partial constriction or tone. 

 In this it may possibly be said to have an automatic action. 

 Though tonic state of activity of the centre may be called auto- 

 matic, it is really under the control of many complex reflex 

 influences, which constantly vary the" general tone, or effect local 



