CHAP, iv.] THE VASCULAR MECHANISM. 273 



or the brain, but it will be desirable to speak of the course of the 

 two sets separately. 



Vaso-constrictor Fibres. In the mammal, so 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 thoracic to the fourth lumbar nerve, both inclu- 

 sive, though some few may pass by the first thoracic and by the 

 fifth lumbar. 



Those for the head and neck leave the spinal cord as we have 

 seen, 144, chiefly by the second and third thoracic nerves, 

 though some leave by the fourth and a variable small number by 

 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 ar- 

 rangement 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 sympa- 

 thetic ganglia (Fig. 72). From thence they reach their destina- 

 tion 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 sympa- 

 thetic ; 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. 72, 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. 23 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 dis- 

 tributed 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 destination, leave the spinal cord by the 



18 



