18 NEKVOUS SYSTEM. 



a definite and thorough knowledge of the functions of these 

 parts, until we have much more positive information con- 

 cerning their anatomical characters. 



Anatomical Divisions of the Nervous Tissue. The phys- 

 iological division of the nervous system into nerves and 

 nerve-centres is pretty well carried out as regards the ana- 

 tomical structure of these parts. The two great divisions of 

 the system, anatomically considered, are into nerve-cells and 

 nerve-fibres. 



The nerve-cells, as far as we know, are the only parts 

 capable, under any circumstances, of generating the nerve- 

 force ; and, as a rule, they cannot receive impressions in any 

 other way than through the nerve-fibres. There are, how- 

 ever, some exceptions, either apparent or real, to this rule, 

 as in the case of direct irritation of the ganglion of the tuber 

 annulare, 'and the sympathetic ganglia, which seem sensible 

 to direct irritation ; but the cells of most of the ganglia be- 

 longing to the great cerebro-spinal axis are insensible to 

 direct stimulation and will only receive impressions con- 

 ducted to them by the nerves. 



The nerve-fibres act only as conductors, and are incapa- 

 ble of generating nerve-force. There is no exception to this 

 rule, but there are differences in the properties of certain 

 fibres. The nerves generally, for example, will receive di- 

 rect impressions, the motor filaments conducting these to 

 the muscles and the sensory filaments conveying the im- 

 pressions to the centres. These fibres will also conduct the 

 force generated by the nerve-centres. But there are many 

 fibres, such as those composing the white matter of the 

 encephalon and the spinal cord, that are insensible to direct 

 irritation, while they will convey to the centres impressions 

 made by the sensitive nerves, and will conduct to the motor 

 nerves stimulus generated by nerve-cells. 



Structure of the Nerves. There are few anatomical ele- 



