x VASCULAE MUSCLE AND NEKVES 349 



constrictor fibres, the origin of which is not yet determined. 

 Thus, e.g., the vessels of the ear also receive constrictors from the 

 second and third nerves of the cervical plexus (Schiff, Loven, etc.). 



The vaso-constrictors of the limbs run principally to the 

 cutaneous vessels, the vessels of the muscles being less well 

 supplied with motor nerves (Sadler, Hafiz, Griitzner and Heiden- 

 hain, and others). 



The vaso-constrictors of the fore-limbs originate in the anterior 

 roots of the median tract of the thoracic cord (third to seventh 

 nerves), join the sympathetic, and unite at the first thoracic 

 ganglion with the ramifications of the brachial plexus. The 

 cervical roots which contribute to the formation of this plexus 

 contain no vaso-constrictor fibres (01. Bernard, Cyon, etc.). Some 

 of these fibres accompany the vessels of the limb directly, without 

 joining the branches of the brachial plexus (Vulpian). 



The vaso-constrictors of the lower limbs have been more 

 studied. They emerge, not from the roots of the lower segment of 

 the cord, but from the thoracic tract and upper segment of the 

 lumbar cord, particularly from the eleventh, twelfth, and thirteenth 

 dorsal, and the first and second lumbar nerves (Bayliss and 

 Bradford). They pass by the rami communicant es to the thoracic 

 and abdominal sympathetic, then, for the most part, joining the 

 great nerve trunks to the limbs, the sciatic especially ; while a few 

 only accompany the vessels of the limbs direct from the abdominal 

 ganglia. 



It is clear from the consensus of observations on the vaso- 

 constrictors in different regions that they originate principally in 

 the anterior roots of the dorsal tract of the cord, pass by way of 

 the rarni communicantes to the ganglion sympathetic system, and 

 thence run directly or indirectly to the vessels, where they form a 

 fine plexus round the muscular tunica media. Their excitability 

 is less, and their latent period longer than that of the motor nerves 

 to the skeletal muscles, and they are constantly in a certain state 

 of activity, on which the tone of the vessels depends. Section of 

 these nerves accordingly produces vascular atony and subsequent 

 hyperaemia, while excitation of the peripheral cord produces 

 constriction or vascular hypertonia and subsequent ischaemia. 



What modifications in local blood pressure and velocity of 

 circulation are produced by such hyperaemia or ischaemia ? Little 

 was known definitely before Dastre and Morat published their 

 observations on the horse. They employed two sphygmoscopes, 

 one applied to the facial artery, the other to the facial vein of the 

 animal. They found that ligation or section of the cervical 

 sympathetic caused a fall of blood pressure in the artery, and a 

 rise in the vein (local circulatory delay). Stimulation of the nerve, 

 on the contrary, produced rise of arterial and fall of venous 

 pressure (local acceleration of circulation), followed by the contrary 



