110 MANUAL OF HISTOLOGY. 



To the naked eye they appear white and glistening, and are 

 the main constituents of the peripheric nerves, though they 

 occur in less number in the sympathetic and also in the brain 

 and cord. Each fibre is made up of three distinct parts : (a) a 

 central cylindrical cord, the axis-cylinder, about which is a (b) 

 coating of soft homogeneous fatty material, called myeline 

 (medulla, white substance of Schwann), forming for the axis- 

 cylinder a sort of tubular sheath, while exterior to both is a 

 delicate membrane or envelope (c\ the sJieath of ScTiwann or 

 primitive sheath. 1 These fibres run a parallel unbranching 

 course, except near their termini or origin, and are surrounded 

 by a connective-tissue coating of varying thickness. Their 

 diameter varies also according to their situation and the degree 

 of their tension or relaxation. In the nerve-trunks the average 

 diameter lies between -fa an ^ iio- millimetre. In the brain they 

 are described as having sometimes a diameter of -g-J-g- jnilli metre, 

 but it is difficult to determine the presence of a medulla in such 

 small fibres. 



To study the properties of a myelinic nerve, we may take a 

 portion of the sciatic from a frog that has just been killed. 

 Having removed it with care and placed it in a drop of water 

 on a slide, we should separate the fibres carefully with needles, 

 taking care not to tear them. Then adjusting a covering 

 glass, it will be seen that from the broken end of the nerve a 

 soft substance is exuding (Fig. 43, 5) ; in a few minutes it is 

 pushed off in the form of drops of irregular shapes (Fig. 43, c). 

 This material is the myeline or medulla. It will be seen to re- 

 fract the light strongly, and show concentric markings. It will 

 also be seen that each fibre has a double contour and is divided 

 at tolerably regular intervals by transverse divisions, which are 

 now known as Ranmer* s nodes. (See Fig. 47.) Midway be- 

 tween each node we may perhaps see an oval body surrounded 

 by a broad expansion of protoplasm. In a few fibres we may 

 even see that a fine thread-like process is projecting from the 

 broken ends of the nerve-fibre the axis-cylinder (Fig. 43, d) 

 while the whole fibre is enclosed by a delicate tightly investing 

 membrane, the sheath of Schwann. Possibly we may also see the 



1 A most unfortunate source of confusion among histologists has arisen from the 

 use of the word neurilemma, which by some is spoken of as synonymous with 

 Schwann 1 s sheath (Frey), and by others as the connective tissue which binds the 

 nerve-fibres together (Klein, Rutherford). We shall avoid the term altogether. 



