506 NERVOUS SYSTEM. 



side over general sensation, the special senses, voluntary and some involun- 

 tary movements, intellection, and, in short, all of the functions that charac- 

 terize the animal. The nerves serve as the conductors of impressions known 

 as general or special sensations and of the stimulus that gives rise to volun- 

 tary and certain involuntary movements, the latter being the automatic 

 movements connected with animal life. 



2. The sympathetic, or organic system. This system is specially con- 

 nected with the functions relating to nutrition, operations which have their 

 analogue in the vegetable kingdom and are sometimes called the functions 

 of vegetative life. Although this system presides over functions entirely 

 distinct from those characteristic of and peculiar to animals, the centres of 

 this system all have an anatomical and physiological connection with the 

 cerebro-spinal nerves. 



The cerebro-spinal system is subdivided into centres presiding over move- 

 ments and ordinary sensation, and centres capable of receiving impressions 

 connected with the special senses, such as sight, audition, olfaction and gusta- 

 tion. The nerves which receive these special impressions and convey them 

 to the appropriate centres are more or less insensible to ordinary impressions. 

 The organs to Avhich these special nerves are distributed are generally of a 

 complex and peculiar structure, and they present accessory parts which are 

 important and essential in the transmission of the special impressions to the 

 terminal branches of the nerves. 



The physiological division of the nervous system into nerves and nerve- 

 centres is carried out as regards the anatomical structure of these parts. The 

 two great divisions of the system, anatomically considered, are into nerve- 

 cells and nerve-fibres. 



The cells of the nerve-centres, while they may transmit impressions and 

 impulses, are the only parts capable, under any circumstances, of generating 

 the nerve-force ; and as a rule, they do not receive impressions in any other 

 way than through the nerve-fibres. There are, however, many exceptions 

 to this rule, as in the case of movements following direct stimulation of the 

 sympathetic ganglia and certain centres in the brain and spinal cord ; but 

 the cells of many of the ganglia belonging to the cerebro-spinal axis are 

 insensible to direct stimulation and can receive only impressions conducted 

 to them by the nerves. 



The nerve-fibres act only as conductors and are incapable 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, receive 

 direct impressions, the motor filaments conducting these to the muscles and 

 the sensory filaments conveying the impressions to the centres. These fibres 

 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 convey to the 

 centres impressions conveyed to them by sensory nerves and conduct to the 

 motor nerves the stimulus generated by nerve-cells. 



In the most natural classification of the nerve-fibres, they are divided into 



