MEDICINES ACTING ON THE NERVOUS SYSTEM 67 



II. The Spinal Cord and Medulla have for their functions 



(1) the control of reflex actions ; and (2) the con- 

 ducting of impulses to and fro between the brain 

 and the various organs and tissues of the body. 

 The cells of the grey matter are chiefly concerned 

 with the former, whilst the white nerve fibres 

 outside them are the conducting media. 



In the medulla, in the floor of the fourth 

 ventricle, are situated the vital reflex centres 

 those concerned with respiration, cardiac inhibi- 

 tion and vaso-motor control also the vomiting 1 

 centre. In the cord itself are centres for micturi- 

 tion, defsecation, parturition, muscular tone, and 

 vaso-motor control. 



These reflex centres of the cord, whilst able of 

 themselves to carry out reflex movements, are 

 more or less under the control of the higher centres 

 of the brain. 



III. Sensory nerves, or afferent nerves, distributed to all 



parts of the body, convey impressions to the brain 

 or cord. 



IV. Motor and secretory or efferent nerves. From the 



cerebro-spinal axis arise nerves which give motion 

 to muscles, and convey other efferent impulses to 

 glands. 



V. Lastly, there are the sympathetic ganglia and nerve 

 fibres, supervising functions such as vaso-motor 

 control, peristalsis, and many others performed by 

 involuntary muscle, and the secretion of some 

 glands. 



Concerning the functions and diseases of the nervous 

 system much has still to be discovered, while the effects 

 of medicines acting upon the brain and cord still require 

 much investigation. 



ACTION OP MEDICINES ON THE BRAIN 



Medicines affecting the brain directly act in one of two 

 ways, either to stimulate or to depress. They pick out, as 

 a rule, some part of the brain as the special seat of action, 



