370 THE NERVOUS SYSTEM. 



material shortly collects into little masses, which distend por- 

 tions of the tubular membrane, while the intermediate spaces 

 collapse, giving the fibres a varicose, or beaded appearance 

 (Fig. 133 c and D), instead of the previous cylindrical form. 



The difference produced in the contents of the nerve-fibres 

 when exposed to the same conditions, has, with other facts, led 

 to the opinion now generally adopted, that the central part or 

 axis-cylinder of each nerve-fibre differs from the outer portion. 

 The outer portion is usually called the medullary or white sub- 

 stance of Schwann, being that to which the peculiar white 

 aspect of cerebro-spinal nerves is principally due. The whole 

 contents of the nerve-tubules appear to be extremely soft, for 

 when subjected to pressure they readily pass from one part of 

 the tubular sheath to another, and often cause a bulging at the 

 side of the membrane. They also readily escape, on pressure, 

 from the extremities of the tubule, in the form of a grumous or 

 granular material. 



That there is an essential difference in chemical composition 

 between the central and circumferential parts of the nerve- 

 fibre, i. e., between the axis-cylinder and the medullary sheath, 

 has of late been clearly shown by Messrs. Lister and Turner. 

 Their observations, founded on Mr. Lockhart Clarke's method 

 of investigating nervous substance by means of chromic acid 

 and carmine, have shown that the axis-cylinder of the nerve- 

 fibre is unaffected by chromic acid, but imbibes carmine with 

 great facility, while the medullary sheath is rendered opaque 

 and brown and laminated by chromic acid, but is entirely un- 

 tinged by the carmine. From this difference in their chemi- 

 cal behavior, the central and circumferential portions of the 

 nerve-fibres are readily distinguished on microscopic examina- 

 tion, the former being indicated by a bright red carmine- 

 colored point, the latter by a pale ring surrounding it. The 

 laminated character of the medullary sheath after treatment 

 with chromic acid is believed by Mr. Lockhart Clarke to be 

 due to corrugations effected by the acid, and not to its having 

 a fibrous structure, as maintained by Stilling. 



The size of the nerve-fibres varies, and the same fibres do 

 not preserve the same diameter through their whole length, 

 being largest in their course within the trunks and branches of 

 the nerves, in which the majority measure from ^oo to sf/ou 

 of an inch in diameter. As they approach the brain or spinal 

 cord, and generally also in the tissues in which they are dis- 

 tributed, they gradually become smaller. In the gray or vesic- 

 ular substance of the brain or spinal cord, they generally do 

 not measure more than from y^Joo to Tfi<r<J f an mcn - 



The fibres of the second kind (Fig. 134), which constitute 



