CHAPTER XXII 



THE VASOMOTOR MECHANISM. SLEEP. THE LYMPHATIC 



SYSTEM 



The Distribution of Blood Among Various Parts of the Body. 



In the nervous control of the heart-beat we have, as already 

 noted, a mechanism whereby the blood-flow through the Body as 

 a whole can be modified in accordance with the needs of the 

 organism. In the vasomotor mechanism we have an arrangement, 

 equally important, whereby individual organs or regions can be 

 furnished with more or less blood as their activities require with- 

 out the necessity of involving the whole circulation. 



The Nerves of the Blood- Vessels. The arteries, as already 

 pointed out, possess a muscular coat composed of fibers arranged 

 around them, so that their contraction can narrow the vessels. 

 This coat is most prominent in the smaller vessels, the arterioles. 

 These vascular muscles are under the control of certain special 

 nerves called vasomotor, and these latter can thus govern the 

 amount of blood reaching any organ at a given time. The vaso- 

 motor nerves belong to the autonomic system. Their physi- 

 ology is therefore the application to special structures of the 

 general principles laid down in connection with that system 

 (Chap. XII). 



In the heart we had to consider a rhythmically contracting 

 organ the force of whose contractions could be increased or dimin- 

 ished by the influence of extrinsic nerves; in the arteries, speak- 

 ing 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 vasoconstrictor nerves, and 

 diminished through the activity of inhibitory or vasodilator 

 nerves. The general tonic contraction of the arterial muscle is, 

 however, much more dependent on the vasoconstrictor nerve- 

 fibers than is the beat of the heart on the cardio-excitor nerves. 

 The inhibitory (dilator) set of vasomotor nerves have a much less 

 extensive distribution over the arterial system than the constrictor. 



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