336 COURSE OF VASO-CONSTRICTOR FIBRES. [BOOK i. 



the fifth and by the first ; those for the fore limbs leave by 

 a number of thoracic nerves reaching from the fourth to the 

 ninth or even the tenth, those by the seventh being the most 

 numerous. Those for the hind limbs leave by the nerves reaching 

 from the eleventh thoracic to the third lumbar, some passing by 

 the tenth thoracic and the fourth lumbar. Those for the tail 

 leave by the first, second and third lumbar. And those for the 

 trunk leave by the successive spinal nerves supplying the trunk. 

 This arrangement may be taken as indicating generally how 

 these fibres leave the spinal cord, bearing, in mind that the fourth 

 lumbar nerve of the dog corresponds to about the second lumbar 

 of man, and that the details differ in different kinds of animals 

 and indeed in different individuals. 



Running in the case of each nerve root to the mixed nerve trunk 

 these vaso-constrictor fibres pass along the visceral branch, white 

 ramus communicans, to the thoracic and abdominal sympathetic 

 ganglia (Fig. 73). From thence 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 ; many of the 

 fibres for the neck however pass directly from the stellate ganglion. 

 Those for the abdominal viscera pass off in a similar way by the 

 splanchnic nerves, Fig. 73, abd. spl. and by smaller nerves joining 

 the inferior mesenteric ganglion. Those destined for the arm, 

 making their way backwards by grey rami communicantes 

 (Fig. 24 r. v.), join 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. These as we have seen are distributed chiefly to the skin, 

 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 destin- 

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

 and then passing through the appropriate visceral branches, join 

 the thoracic or abdominal sympathetic ganglia. In their course the 

 fibres undergo a remarkable change. Along the anterior root and 

 along the visceral branch they are medullated fibres, but before 

 they reach the blood vessels for which they are destined they 

 become non-medullated fibres ; they appear to lose their medulla 

 in some or other of the ganglia. 



We are in many cases able to determine experimentally by the 

 following method, the ganglion or ganglia in which particular 

 fibres end, that is to say in which they become connected with 

 nerve cells. It is found that the drug nicotin abolishes or 

 suspends the action of vaso-motor fibres and of other fibres 

 running in the sympathetic system. Thus in a rabbit, after a 

 certain dose of nicotin has been given, stimulation of the cervical 

 sympathetic nerve in the neck no longer causes constriction of 



