68 ACTION OF MEDICINES ON THE BRAIN 



but gradually the effect tends to spread to all parts of the 

 central nervous system. How the effects are produced is 

 not easy to decide, but there are various modes of action 

 which are of importance. Some drugs are absorbed easily, 

 and without any chemical change are readily soluble in the 

 fatty material of the brain, whilst being relatively insoluble 

 in water. All these act as narcotics. Others, again, chemi- 

 cally combine with the nerve cells, for example the toxin 

 of tetanus, which unites with the cells of the spinal cord to 

 form a non-toxic inert substance. Still others act on nerve 

 cells by their osmotic effects, such as salines circulating in 

 the blood. 



Thus, by various slight physical and chemical changes 

 the cells of the nervous system are affected, and, on the one 

 hand, may become quicker to respond to excitation (stimu- 

 lated), or less quick to respond (depression), and so less able 

 to perform their functions. 



The motor areas of the cerebral cortex have their excita- 

 bility lowered by alcohol, chloral, and cold. The depression 

 caused by cold, unless extreme, or applied for a long period, 

 is followed, however, by reaction. Bromides of potassium 

 and ammonium, without disturbing the relations of one 

 centre to another, appear to have a marked effect in lower- 

 ing general brain activity. Still more prompt and powerful 

 are anaesthetics which abolish all motor action. Atropine 

 in moderate doses at first increases, but in large doses 

 diminishes, motor excitability. 



The motor centres have their excitability increased by 

 mechanical irritation, as by the point of a needle, which pro- 

 duces epileptic convulsions. Similar convulsions also ensue 

 when the vessels of the brain are surcharged with venous 

 blood, as in asphyxia, but these are medullary in origin, 

 from the convulsive centre related to the respiratory centre. 

 Camphor and other volatile oils cause excitement and con- 

 stant movements, succeeded, after large doses, by clonic 

 convulsions and death. The active principles of cocculus 

 indicus, cicuta virosa, and oenantha crocata, as well as 

 cinchonidine and quinine, have similar convulsant effects. 

 The action of these agents is not confined to the brain motor 

 centres, but also extends to those in the medulla. 



