THE NERVOUS SYSTEM IN GENERAL. 655 



bundle to which they are distributed. This disposition has given rise to the 

 opinion that every nerve-fibre, no matter how long its course may be, is a 

 loop that originates in, and returns to, the same cell. One of the fibres that 

 enters the cell, and looks like a stalk to it, is usually straight ; the other 

 seems to arise from the outside of the cell, is sometimes double, and circles 

 round the other in a spiral manner two or three times. Both fibres at first 

 resemble the cylinder-axis of ordinary nerve-fibres, and may subsequently be- 

 come dark-bordered, or remain pale fibres. The spiral fibres bear large oblong 

 nuclei in their course. Some observers state that they have traced the 

 straight fibre into the nucleus, and the spiral fibre into a plexus on the 

 exterior of the cell, but which may be ultimately traced into the nucleolus ; 

 so that the two fibres are continuous through the nucleus and nucleolus). 



PROPERTIES AND FUNCTIONS OF THE NERVOUS SYSTEM. 



It would require a long chapter to do justice to this subject, and we 

 could not venture on it here without going beyond our domain. We will, 

 however, offer some remarks on those notions connected with the properties 

 and functions of the nervous system which are strictly necessary for the 

 comprehension of the anatomical facts to be hereafter dealt with. 

 And first as to the properties of the nerves. 



We will suppose the spinal canal to be opened in the lumbar region, and 

 he cord laid bare in a living animal. If we cut across the inferior roots of 

 >ne of the spinal nerves, and if we compress with a pair of forceps one or more 

 >f these roots, by the end remaining attached to the cord, nothing results 



denote that this irritation has had any influence on the organism. But 

 f, instead of operating on the central or attached end of these divided roots, 

 ve excite the peripheral end which is continued by the trunk of the nerve, 

 ontraction of the muscles of the limb which receives the fibres coming 

 rom the irritated roots is produced. 



The muscular tissue comports itself as if the irritation were directly 

 ipplied to it ; so that the nerve has served as the medium of communication. 



1 has received the excitation, it has been impressioned by it, and it has 

 xmducted this to the muscles to which the nerve is distributed. This 

 Louble reaction produced by the nervous tubes is their special attribute, 

 ,heir essential property. With M. Vulpian we might designate it as a whole 

 ay the name of neunlity ; but it is necessary to distinguish the two modes it 

 iffects by naming the property of being impressed by excitations as the 

 excitability of the nerve, and nervous conductivity its aptitude to convey the 

 excitations which have impressed it. 



The same experiment may be repeated on the upper roots. It is then 

 )erceived that the pinching, which produces no effect at the peripheral 

 Bxtremity, causes pain when applied to the central end. The animal 

 estifies immediately, by cries and movements, that it feels the touch of the 

 brceps. But, as will be mentioned in a moment, the impression resulting 

 rom this touch has only been perceived by the brain ; it has therefore been 

 conducted to the spinal cord by the excited nervous fibres, and then to the 

 jrain by the fibres of this medullary axis. 



In putting to one side, for the moment, the part played by the latter in 

 the phenomenon now analysed, it will be seen that the superior fibres of the 

 spinal nerves enjoy the same attributes as the inferior ; neurilily is their 

 appanage, and this property is apparent in its two qualities excitability and 

 conductibility. Only here the latter .property is exercised in a centripetal 



