FUNCTIONS OF NEEVE-FIBRES. 



431 



the nerve-fibres, we would submit that the neurilemma is originally 

 for the development of the contained parts ; the medulla protects 

 and isolates the axis-fibre ; while the latter is endowed with the pe- 

 culiar vital property of the fibre, whether it be efferent or afferent. 

 Thus the motor fibres, being, most exposed to undergo pressure 

 among the muscular fibres while contracting, need a thicker me- 

 dulla, and are therefore coarser. The fibres of the ganglionic nerves 

 are at the other extreme in this respect ; since they are mostly dis- 

 tributed to internal parts and organs, and at the same time follow 

 vessels more especially. It also occurs, that a thick medulla at the 

 distal extremity of a sensory fibre would of course interfere with a 

 prompt impressibility of the axis-fibre, though it might become 

 medullated while lying in the nerve-trunks. Evidently, also, the 

 protection of a medulla is not required after the fibres enter the 

 substance of the encephalon and spinal cord; and here the fibres 

 are non-medullated. 



For the present, therefore, we are not perhaps allowed to assume 

 more than that a fibre which is thickly medullated throughout its 

 entire course, is very probably a motor fibre. But some of the finer 

 fibres may also be motor. The finest fibres are not peculiar to the 

 ganglionic nerves ; and if they actually manifest a peculiar function 

 (i. e. as the great sympathetic), that function is probably manifested 

 by the spinal nerves also, in proportion to the fine fibres they 

 contain. 



Development of the Nerve-Fibres. 



The peripheral portions of the nerve-fibres are developed inde- 

 pendently of the central portions; but in such a way that the lat- 



ie 



1. Two nerve-fibres from the ischiatic nerve of a sixteen weeks' embryo. 2. Nerve-tubes from a 

 newly littered rabbit, a. Neurilemma. 6. Nucleus, c. Medullary sheath. 3. Nerve-fibre from the tail 

 of the tadpole (a, 5, e), as before ; at d, the fibre retains its embryonic character ; the dark bordered 

 fibre shows a division. (Kolliker.) 



