PLAN OF .THE NERVOUS SYSTEM. 493 



irritable, or capable of being brought into action by an im- 

 pression from without. It can also originate action. It is 

 automatic as well as reflective. The automatic action of the 

 higher centres is the basis of the emotions, of the intellect, and 

 of the will, and is continually modifying the impulses flowing 

 out of the reflex centres, and vice versa, by means of connecting 

 fibres or tracts. In the same way the viscera, such as the 

 heart, are innervated by automatic centres in the medulla or 

 cord, and these are constantly influenced by impressions 

 reaching them from all sides. The highest centres are in the 

 convolutions ; the simple automatic reflex centres in the basal 

 ganglia, cerebellum, medulla, and cord the whole constituting 

 a series of successive centres, the central nervous system, joined 

 to each other by tracts which associate or co-ordinate the im- 

 pulses, whilst the ouilpng systems of ganglia, chiefly automatic 

 in their action, are called the sympathetic. 



Now we find, when we come to consider the action of drugs 

 and other remedies on the nervous system, that certain of 

 them affect one centre, some another ; some afferent parts, others 

 efferent or motor parts ; that some drugs affect the lower 

 centres only some the centres of emotion and intelligence only; 

 and that others again interfere chiefly with the co-ordinating 

 mechanism. We must therefore attempt to arrange the parts 

 of the nervous system on something like a definite plan, before 

 we can comprehend the action of drugs upon it. 



Plan of the Nervous System. 



I. The terminal irritable apparatus, on the surfaces of 



the body, and in the organs. 

 II. The afferent nerves. 



III. The posterior of cornua of the cord. 



IV. The convolutions. 



V. The basal ganglia and cerebellum. 

 VI. The medulla oblongata. 

 VII. The antero-lateral tracts and anterior cornua of the 



cord. 



VIII. The co-ordinating fibres between the different centres, 

 especially in the cord, where they form definite 

 columns. 



If we were to add to this plan, we might put in the viscera 

 with their nerves. These we have relegated to other chapters ; 

 and all that need be indicated at present is that most of the 

 viscera are governed by centres in the medulla, cord, or cere- 

 brum, an arrangement which is partly reflex : that the efferent 

 nerves between the centres and the viscera are intimately con- 



