VASO-MOTOR NERVES AND NERVE-CENTRES. 275 



large number of arteries, the dilatation of the latter so much 

 diminishes peripheral resistance to the blood-flow as to lead 

 to a marked fall of general arterial pressure; and, due care 

 being taken to avoid or to allow for concomitant variations in 

 the rate or force of the heart's beat, this gives us another use- 

 ful method of studying the distribution of the nerves con- 

 cerned. For example, the splanchnic nerves are branches 

 which spring from the thoracic portion of the sympathetic 

 chain and pass through the diaphragm to end in the gan- 

 gliated solar plexus from which nerves pass to the arteries of 

 most of the abdominal viscera. When the splanchnic nerves 

 are cut on both sides arterial pressure falls enormously, from 

 say 120 millimetres of mercury in the carotid of a dog to 15 

 or 20 millimetres, most of the blood of the body lying almost 

 stagnant in the dilated blood-vessels of the abdomen. On the 

 other hand, stimulation of the splanchnic nerves so diminishes 

 the paths open for the circulation of the blood as to enor- 

 mously increase general blood-pressure; especially if the 

 cardio-inhibitory nerves be first divided so that raised blood- 

 pressure inside the skull-chamber may not slow the heart- 

 beat. 



The skin and the abdominal organs seem to be the pre- 

 dominant localities of distribution of the vaso-constrictor 

 nerves: other parts have them, but not in quantity sufficient 

 to bring about any great general change in the blood-flow. 

 In the abdomen is warmer, in the skin cooler blood: and 

 according to the amount of heat produced in the Body and 

 the temperature of the surrounding medium, the vessels of 

 abdomen and skin contract or relax so as to control the pro- 

 portion of blood sent to the skin to lose heat. 



The Vaso-constrictor Centre. The constrictor nerves of 

 the arteries do not originate in the sympathetic system. If 

 all the branches of the latter be left intact, the phenomena of 

 paralytic dilatation of the blood-vessels can be fully brought 

 about by dividing the communicating branches between certain 

 spinal nerves and the corresponding sympathetic ganglia, 

 or by dividing the anterior roots of certain spinal nerves. 

 In this way it can be shown that the fibres all proceed 

 from the thoracic and lumbar regions of the spinal cord, 

 but have not their origin in the cord. If it be cut anywhere 

 in the cervical region, all arteries having a constrictor nerve 

 supply are paralyzed, while stimulation of the posterior end 



