176 PALE NEKVE-FIBRES OF THE SYMPATHETIC SYSTEM. 



234. Of the pale nerve-fibres of the sympathetic 

 system. In cases in which the distance between the 

 nerve centre and the peripheral distribution of the 

 nerves is not very great, the compound fibres are not 

 insulated by a " medullary sheath." In many of the 

 nerve fibres belonging to the so-called sympathetic 

 system there is no " medullary sheath," or " white 

 substance of Schwann." Where, however, the 

 ganglia or peripheral organs are connected witli 

 nerve centres at a considerable distance off, a number 

 of fibres having this investment are found ; so tha t 

 amongst the sympathetic nerve fibres, we find dark- 

 bordered nerve-fibres just as commonly as pale fibres 

 occur in the trunks of spinal dark- bordered nerve- 

 fibres ( 235). In the bladder of the frog I have ob- 

 served that where the distance between the ganglion 

 and the peripheral distribution of the nerve fibres is 

 considerable, the fibres have the dark-bordered cha- 

 racter while, on the other hand, if the peripheral dis- 

 tribution is near the ganglion, the ultimate nerve-fibres 

 are connected with the latter by pale fibres only. 



235. Of fine fibres running with the dark-bordered 

 nerve-fibres. I have described and figured very fine 

 nerve-fibres running close to the dark-bordered 

 nerve-fibre in the same sheath with it, when there 

 exists a sheath, and in the bundles of dark-bordered 

 nerve-fibres near their distribution, several such fine 

 fibres may be discerned. These fine fibres themselves 

 result from the division of dark-bordered nerve-fibres. 

 Sometimes, a dark-bordered nerve-fibre divides into 

 one dark-bordered, and one fine fibre. The fine fibres are 

 in fact the continuation of the dark-bordered fibres, and 

 are very near the point of ultimate distribution. These 

 fine fibres accompanying dark-bordered nerve fibres, 

 have not been described or figured by other ob- 

 servers, and are quite undemonstratable in specimens 

 prepared according to the methods usually followed, 

 though they are very distinct indeed in specimens, 



