MEDULLATED NERVE-FIBRES. 



309 



may be divided into three sets or kinds according to their size. Thus, most 

 fibres which are found in the ordinary or cerebro -spinal nerves are from 8/j. to 

 16/i in diameter ; whereas those which pass from the roots of the spinal nerves 

 to the sympathetic average only from 1'8/i to 3'6/n, while fibres of an inter- 

 mediate size occur in large numbers in the vagus, the glosso-pharyngeal, the facial, 

 and in the motor root of the fifth nerve as well as in the anterior roots of the spinal 

 nerves (Gaskell). These differences are illustrated in fig. 384, p. 329. 



Many of the medullated nerve-fibres appear dilated or swollen out at short distances along 

 their length, and contracted in the intervals between the dilated parts. These fibres, however, 

 are naturally cylindrical like the rest, and continue so while they remain undisturbed in their 

 place ; and the varicose character is occasioned by pressure or traction during the manipula- 

 tion, which causes the soft matter to accumulate at certain points, whilst it is drawn out and 

 attenuated at others (fig. 354), The fibres in which this is most apt to occur are usually of 

 small size, ranging from -^^ to j^gth of an inch in diameter ; and when a very small fibre is 

 thus affected, the varicosities appear like a string of globules held together by a fine transparent 

 thread. 



Structure of medullated fibres. The medullated fibres are composed for the 

 most part of three distinct structures, viz., an axial fibre (the axis-cylinder of 



Fig. 355. DIAGRAM to show the parts of a medullated fibre, viz., 1, 1, outer or primitive 

 tlieath enclosing the doubly contoured white substance or medullary sheath. 2, a part where the white 

 substance is interrupted, the outer sheath remaining. 3, axis cylinder projecting beyond the broken end 

 of the tube. 4, part of the contents of the tube escaped. 



Fig. 356. Two PORTIONS OF MEDULLATED NERVE FIBRES, AFTER TREATMENT WITH OSMIO ACID, 



SHOWING THE AXIS-CYLINDER, AND THE MEDULLARY AND PRIMITIVE SHEATHS (Key and Retzius). 



A. Node of Ranvier. B. Middle of an internode with nucleus. 



c, axis-cylinder, projecting at the broken end ; p, primitive sheath within which the medullary sheath, 

 which is stained dark by the osmic acid, is somewhat retracted. 



Fig. 357. PART OP AN AXIS-CYLINDER, HIGHLY MAGNIFIED, SHOWING THE FINE VARICOSE FIBRILS 



WITHIN IT (Max Schultze). 



Purkinje), enclosed within two sheaths, one of these being the medullary sheath 

 already mentioned, and the other a delicate membranous tube outside of all, termed 

 the nucleated sheath of Schwann, the primitive sheath, or the neurolemma. 1 But there 

 are medullated fibres in which the primitive sheath is absent, and other fibres and 

 prolongations of fibres in which there is no. sheath whatever to the axis-cylinder. 



1 The term neurilemma or neurolemma was formerly applied to the connective tissue sheath of the 

 funiculus (see p. 325), which is now known as the perineurium. 



