116 STRUCTURE OF A NERVE FIBRE. [BOOK i. 



whole fibre becomes more transparent ; and if such a fibre, either 

 before or after the treatment with ether, be stained with carmine 

 or other dye, the axis cylinder will be seen as a stained band 

 or thread lying in the axis of a tubular space defined by the 

 neurilemma which stains only slightly except at and around the 

 nuclei which, as we have seen, are embedded in it at intervals. 

 In the entire fibre the tubular space between the axis cylinder 

 and the sheath is filled with a fatty material, the medulla, which 

 from its fatty nature has such a refractive power as to exhibit a 

 double contour when seen with transmitted light, on which 

 account the fibre itself has a double contour. It is this refractive 

 power of the medulla which gives to a nerve fibre and still more 

 so to a bundle of nerve fibres or to a whole nerve a characteristic 

 opaque white colour when viewed by reflected light. 



As we shall see, all nerve fibres do not possess a medulla, and 

 hence such a fibre as we are describing is called a medullated 

 Jibre. 



A typical medullated fibre consists then of the following parts. 



1. The axis cylinder, a central cylindrical core of so called 

 'protoplasmic' material, delicate in nature and readily undergoing 

 change, sometimes swelling out, sometimes shrinking, and hence 

 in various specimens appearing now as a thick band, now as a thin 

 streak in the axis of the tubular sheath, and giving in cross section 

 sometimes a circular, sometimes an oval, and not unfrequently a 

 quite irregular outline. Probably in a perfectly natural condition 

 it occupies about one half the diameter of the nerve, but even its 

 natural size varies in different nerve fibres. When seen quite 

 fresh it has simply a dim cloudy or at most a faintly granular 

 appearance ; under the influence of reagents it is apt to become 

 fibrillated longitudinally, and has been supposed to be in reality 

 composed of a number of delicate longitudinal fibrill^ united by 

 an interfibrillar substance, but this is not certain. It is further 

 said to be protected on its outside by a transparent sheath, the 

 axis cylinder sheath, but this also is disputed. 



The axis cylinder passes unbroken through successive nodes of 

 Ranvier, the constriction of the node not affecting it otherwise 

 than perhaps to narrow it. Now the fibres of a spinal nerve 

 (omitting for the present the fibres coming from the sympathetic 

 nerves) may be traced back either to the spinal ganglion on the 

 posterior root, or along the anterior root to the anterior cornua 

 of the spinal cord ; and as we shall see the axis cylinders of the 

 fibres are, in both cases, prolongations of processes of nerve cells, 

 in the former case of cells of the ganglion, in the latter case of 

 cells of the anterior cornua. In each case a process of a cell 

 becoming the axis cylinder of a nerve fibre runs an unbroken 

 course, passes as a continuous band of peculiar living matter, 

 through node after node right down to the termination of the fibre 

 in the tissue in which the fibre ends ; the only obvious change which 



