i 9 4 PHYSIOLOGY FOR BEGINNERS CHAP, xvn 



back into the spinal nerves and others which pass into some 

 of the cranial nerves ; these are thus distributed to the blood- 

 vessels of the limbs, trunk, and other parts to which the spinal 

 or cranial nerves go. The sympathetic nerves chiefly carry 

 impulses which govern the muscular tissue of the viscera and 

 the muscular coat of the small arteries of the various tissues. 

 It is through the sympathetic nerves that the tone of the 

 blood-vessels is kept up by the action of the vaso-motor centre 

 in the spinal bulb. It must be clearly understood that the 

 sympathetic derives the impulses which it distributes from the 

 central nervous system ; these do not arise in the sympathetic 

 itself. The impulses come out of the spinal cord by the 

 anterior roots of the spinal nerves, .and so pass by the short 

 branches, spoken of above, into the sympathetic chains. 



Very many of the nerve fibres which run in the branches 

 of the sympathetic chains are non-medullated fibres. 



