CHAPTER XVIII. 

 THE VASO-MOTOR NERVES AND NERVE-CENTRES. 



The Nerves of the Blood-vessels. The arteries, as 

 already pointed out, possess a muscular coat composed of 

 fibres arranged around them, so that their contraction can 

 narrow the vessels. This coat is most prominent in the 

 smaller vessels, those of the size which go to supply separate 

 organs, but disappears again in the smallest branches, which 

 are about to divide into capillaries for the individual tissue 

 elements of an organ. These vascular muscles are under the 

 control of certain special nerves called vaso-motor, and these 

 latter can thus govern the amount of blood reaching any 

 organ at a given time. Most of the vascular nerve-fibres 

 have their origin in the cerebro-spinal centre, though they 

 pass through sympathetic ganglia on their way to the vessels. 

 In a few regions ganglion-cells are found lying close to the 

 arteries, and some of the vaso-motor fibres are probably con- 

 nected with them, but as a rule they end directly in the mus- 

 cular coat. 



In the heart we had to consider a rhythmically contract- 

 ing organ the force of whose contractions could be increased 

 or diminished by the influence of extrinsic nerves; in the 

 arteries, speaking broadly, we have to deal with muscle in a 

 condition of tonic or constant contraction, which contraction 

 can be increased by impulses coming through excitor or vaso- 

 constrictor nerves, and diminished through the activity of 

 inhibitory or vaso-dilator nerves. The general tonic con- 

 traction of the arterial muscle is, however, much more de- 

 pendent on the vaso-constrictor nerve-fibres than is the beat 

 of the heart on the cardio-excitor nerves. The inhibitory 

 set of vaso-motor nerves have a much less extensive distribu- 

 tion over the arterial system than the constrictor. 



The Vaso-constrictor Nerves. If the ear of a white rab- 

 bit be held up against the light while the animal is kept quiet 

 and not alarmed, the red central artery can be seen coursing 



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