7 6 HUMAN PHYSIOLOGY 



(a) The vaso-constrictor nerves have their centre in the 

 medulla oblongata, extending from the upper part of the 

 fourth ventricle to the lower part of the calamus scriptorius, 

 on both sides. From this centre the nerve-fibres proceed 

 down the spinal cord and connect with the nerve-cells of the 

 gray matter ; from there the fibres pass through the anterior 

 roots and the rami communicantes into the sympathetic. The 

 sympathetic fibres proceed separately (e.g. splanchnic) or 

 with other peripheral nerves (e.g. trigeminus, sciatic) to the 

 blood vessels. Some vaso-motor fibres go directly to the 

 vessels without passing through the sympathetic (e.g. from 

 the roots of the lower lumbar and of the sacral nerves). 

 The vaso-motor nerves are perhaps not in direct contact with 

 muscle fibres, but pass first into the ganglionic cells of the 

 walls, from which the motor fibres proceed to the muscles. 



The vaso-motor centre is tonic. Section of a vaso-motor 

 nerve causes a dilation of the vessels innervated by that 

 nerve. 



If the cord is cut, dilation of the vessels supplied by the 

 sectioned vaso-motor nerves results; but eventually the tonus 

 is regained, evidently because the cells in the spinal cord, 

 through which the vaso-motor nerves pass, have assumed 

 the function of the centre. Even after section of a peripheral 

 vaso-motor nerve, the tonus is eventually regained ; in this 

 case, the ganglionic cells in the walls of the blood vessels 

 assume the role of the centre. 



The activity of the vaso-motor centre is influenced : 



1. Directly by lack of oxygen and the accumulation of 

 carbon dioxide in the blood ; this increases its action. 

 Hence asphyxia stimulates this as it does the cardiac inhibi- 

 tory centre. 



2. By stimuli conducted to it through the nerves. 



(a) Psychical processes can increase or diminish its. 

 activity (pallor by fear; blushing). 



(b} The activity can be influenced reflexively. 

 We classify the centripetal nerves used in this reflex 

 action into: 



