CHAP, iv.] THE VASCULAR MECHANISM. 317 



nerve, the fibres being either vaso-constrictor or vasodilator, and 

 some nerves containing one kind of fibres only, some both in vary- 

 ing proportion. Almost every nerve in the body therefore may 

 be looked upon as influencing a certain set of blood vessels, as 

 governing a vascular area, the area being large or small, and the 

 government being exclusively constrictor or exclusively dilator or 

 mixed. 



The Course of Vaso-constrictor and Vaso-dilator Fibres. 



169. Both the vaso-constrictor and the vaso-dilator fibres 

 have their origin in the central nervous system, the spinal cord 

 or the brain, but the course of the two sets appears to 'be very 

 different. 



In the mammal, as far as we know at present, all the vaso- 

 constrictor fibres for the whole body take their origin in the 

 middle region of the spinal cord, or rather, leave the spinal cord 

 by the nerves belonging to this middle region. Thus in the dog 

 the vaso-constrictor fibres, not only for the trunk but for the 

 limbs, head, face and tail, leave the spinal cord by the anterior 

 roots of the spinal nerves reaching from about the second dorsal 

 to the second lumbar nerve, both inclusive. Running in the case 

 of each nerve root to the mixed nerve trunk they pass along ^ 

 the visceral branch, white ramus communicans, to the chain of 

 splanchnic ganglia lying in the thorax and abdomen, the so-called 

 thoracic and abdominal sympathetic chain (Fig. 58). From these 

 ganglia they reach their destination in various ways. Thus, those 

 going to the head and neck pass upward through the annulus of 

 Vieussens to the lower cervical ganglion and thence, as we have 

 seen, up the cervical sympathetic. Those for the abdominal 

 viscera pass off in a similar way to the abdominal splanchnic 

 nerves, Fig. 58, abd. spl. Those destined for the arm take their way 

 by the recurrent fibres (grey rami communicantes), (Fig. 24 r. v.), 

 and so reach the nerves of the brachial plexus; while those for 

 the hind leg pass in a similar way through some portion of the 

 abdominal sympathetic before they join the nerves of the sciatic 

 plexus. And the constrictor fibres of the skin of the trunk probably 

 reach the spinal nerves in which they ultimately run in a similar 

 manner. All the vaso-constrictor fibres, whatever their destina- 

 tion, leave the spinal cord by the anterior roots of spinal nerves, 

 and then passing through the appropriate visceral branches, join 

 the thoracic or abdominal chain of splanchnic ganglia. In these 

 ganglia the fibres undergo a remarkable change. Along the 

 anterior root and along the visceral branch they are medullated 

 fibres, but long before they reach the blood vessels for which they 

 are destined they become non-medullated fibres ; they appear to 

 lose their medulla in the system of splanchnic ganglia. We may 

 add that in the anterior roots and along the visceral branches, 



