NERVOUS ACTION. 31Ki 



times the fibres seem to loose their dark outline and white 

 substance, become less distinct, and are ultimately lost in 

 the substance of the tissue ; 3d, In some cases the ultimate 

 nerve-fibres seem resolved into minute plexuses, as in some 

 of the serous membranes ; 4th, Sometimes they terminate 

 in free ends which may enter minute ovoid bodies Pad- 

 man bodies met with at certain parts of the surface of the 

 true skin ; 5th and lastly, they may terminate, as in the 

 eye and ear, by becoming connected with true nerve-cells. 

 The presence of such cells, however, constitutes the part a 

 true nervous centre. 



Nerve-fibres, we have said, are conductors of impressions 

 and of nervous force. It is remarkable that in the per- 

 formance of these different acts special fibres are employed, 

 and this remark applies equally to the cerebro-spinal and 

 sympathetic systems. Thus, in both alike there are certain 

 fibres which conduct impressions only towards the nervous 

 centre (sensory, afferent, or centripetal), and others conduct 

 only motor force (efferent or centrifugal). No nerve fibre 

 can convey more than one kind of impression. There is no 

 difference in the size or appearance of these fibres to indi- 

 cate their special functions, so that their relative properties 

 can only be ascertained by observing their action under the 

 influence of a stimulus. This is ordinarily supplied to sen- 

 sory nerves by external objects applied to their extremities, 

 and to motor by the will, or some reflex stimulus through 

 the medium of a nervous centre. Any stimulus, however, 

 applied to the trunk of a nerve, is sufficient to bring it into 

 a state of activity ; and it does not import whether this' 

 stimulus is mechanical, chemical, electrical, or a simple 

 excess of heat or cold. The application of any such ex- 

 citant to the trunk of a sensory nerve leads to the idea of 

 pain, &c., not only in the irritated point, but also in those 



