clxii NERVOUS SYSTEM. 



Among various physiologists of consideration, who adopt this view in a 

 more or less modified shape, some have been further of opinion that the 

 fibres of ganglionic origin differ iu structure, size, and other physical cha- 

 racters from those which arise in the cerebro-spinal axis. As regards this 

 question, I must confess, that there does not seem to me to be conclusive 

 evidence to show that peculiar anatomical characters are distinctive of the 

 fibres of different origin. It has been already stated that both dark- 

 bordered and pale fibres may be connected with ganglion-cells, and for auyht 

 that has been proved to the contrary, all three varieties of fibres spoken of, 

 large tubular, small tubular, and grsy, may arise both in the cerebro-spinal 

 axis and in the ganglia ; although it is certainly true that the two latter 

 kinds largely predominate in the sympathetic, and abound in other nerves, or 

 branches of nerves, which appear to receive large contributions from ganglia. 



VITAL PROPERTIES OF THE NERVOUS SYSTEM. 



The fibres of nerves are endowed with the property of transmitting im- 

 pressions, or the effect of impressions, from the point stimulated towards 

 their central or their peripheral extremities. One class of fibres conduct 

 towards the nervous centres and are named " afferent," their impressions 

 being " centripetal ; " another class of fibres conduct towards their distal 

 extremities, which are distributed in moving parts, arid these fibres are 

 named " efferent," whilst their impressions are " centrifugal." Impres- 

 sions propagated centripetally along the nerves to the brain give rise to sen- 

 sations, varying according to the nerve impressed, and the objective cause 

 of the impression ; stimuli transmitted outwardly, on the other hand, are 

 conveyed to muscles, and excite movements. Motorial stimuli thus passing 

 along efferent nervous fibres may emanate from the cerebrum as in volun- 

 tary and emotional movements, or possibly from some other central part, as 

 in the case of certain involuntary motions ; or such stimuli may be applied 

 in the first instance to afferent fibres, by these conducted to the brain or 

 some other central organ, and then " reflected " by the central organ to 

 efferent fibres, along which they are propagated to the muscle or muscles to 

 be moved ; and in this case the intervention of the central organ may give 

 rise to sensation or not, the difference in this respect probably depending on 

 the part of the nervous centre where the reflection takes place. 



Ihe property of conducting a stimulus or propagating its effects in a deter- 

 minate direction, belongs to the fibres of the nerves, and in all probability 

 also to the fibrous part of the nervous centre, while it is probable that to 

 the cells or corpuscles of the grey matter of the central organs, is assigned 

 the office of receiving impressions conveyed from without, and presenting 

 them to the conscious mind, of mediating between the mind and the efferent 

 fibres in excitation of the latter by mental stimuli (as in voluntary and 

 emotional acts), of transferring to efferent fibres stimuli conducted to the 

 centre by afferent fibres in the production of reflex movements, and, pos- 

 sibly, of originating purely corporeal stimuli in certain involuntary motions. 

 In addition to these endowments, the nerves are concerned in controlling 

 and regulating the molecular changes and chemical actions which occur in 

 nutrition, secretion, and other allied processes. It may no doubt be fairly 

 questioned, whether the effect justly attributable to the nerves in such cases 

 is not produced merely through the influence which they exert over the 

 motions of the minute vessels and contractile tissues concerned ; but the 

 tendency of late observations on the nerves of secreting glands, and of the 

 experiments on the luminiferous organ of the fire -fly, referred to in a 



