CHAP. XVIII.] THE NERVOUS TISSUE. 



201 



6 c d e / 

 FIG. 116. NERVE-FIBRES. 



3 



a, nerve- 



of white substance, and called medullated nerve-fibres ; and those 

 having no white sheath, called non-medullated nerve-fibres. 



The medullated nerve-fibres consist of three parts: (1) the 

 axis-cylinder; (2) the white, or medullary sheath; (3) the 

 neurilemma, or enclosin-g sheath. 



(1) The axis-cylinder is the essential part of every nerve- 

 fibre, and runs as a somewhat indistinct strand in the axis of 

 the fibre, and it is sometimes 



called the axis-band. It ap- 

 pears to be in every case a 

 direct prolongation of a branch 

 of a nerve-cell, and may there- 

 fore be looked upon as a far- 

 extending cell -process. It 

 passes without any break or 

 interruption from the nerve- fibre> showing complete interruption of 



Centres to the periphery; that the white substance; b, another nerve- 



. . . fibre with nucleus. In both these nerve- 



1S to Say, it IS continuous trom fibres the white substa nce is stained black 



end to end, from its Origin to withosmic acid, and the axis-cylinder is 



. seen running as an uninterrupted strand 



Its termination. through the centre of fibre, c, ordinary 



(2) The medullary sheath nerve-fibre unstained ; d, e, smaller nerve- 



fibre ; /, varicose nerve-fibre ; g, non-me- 



surrounding the axis-cylinder duiiated nerve-fibres. 

 is a fatt}r, semi-solid, white sub- 

 stance, which does not form a complete tube, but at tolerably 

 regular intervals is separated into segments ; it is not continu- 

 ous from end to end like the axis-cylinder, being wanting at 

 the origin and termination of the nerve-fibre. (We may look 

 upon the axis-cylinder as'the live or naked wire of the fibre, and 

 the white sheath as the isolating, non-conducting substance.) 



(3) The neurilemma or enclosing sheath is a thin structure- 

 less tube, which tightly encloses the medullary sheath as the 

 sarcolemma encloses the muscle-fibre. 



The non-medullated fibres have no medullary sheath and no 

 neurilemma. They frequently branch as the medullated fibres 

 never do except near their termination. 



The nerve-fibres are gathered into cords of variable size to 

 form the nerves. These cords are called funiculi. Each f uni- 

 culus has its own sheath of connective tissue. If a nerve is 

 small, it may consist of one funiculus, but in larger nerves sev- 

 eral funiculi are united by connective tissue into one large cord 



