508 



THE NEKVOUS SYSTEM. 



FIG. 448. Two MULTIPOLAR NERVE- 

 CELLS (from a specimen prepared by 

 the Golgi method). 



view, therefore, four different 

 forms of nerve-fibre may be 

 recognised : 



Non-med ullated 



1. Naked axis-cylinders. 



2. Axis -cylinders with primi- 



tive sheaths. 

 Medullated 



3. Primitive sheath absent. 



4. Primitive sheath present. 



Every nerve -fibre near its 

 origin and as it approaches its 

 termination is unprovided with 

 sheaths of any kind, and is 

 simply represented by a non- 

 medullated, naked axis- 

 cylinder. The fibres of the 

 olfactory nerves afford us an 

 example of non - medullated 

 fibres furnished with a primi- 

 tive sheath. 



Medullated fibres are 

 present in greater quantity in 

 the cerebro-spinal system than 

 non -medullated fibres. Inus, 

 all the nerves attached to the 



Nerve - fibres. Nerve - fibres, ar- 

 ranged in bundles of greater or less bulk, 

 form the nerves which pervade every 

 part of the body. They also constitute 

 the greater part of the brain and spinal 

 medulla. Nerve-fibres are the conduct- 

 ing elements of the nervous system ; they 

 serve to bring the nerve -cells into 

 relation both with each other and with 

 the various tissues of the body. 



There are different varieties of nerve- 

 fibres, but in all the leading and essential 

 constituent is a delicate thread-like axon. 

 The most obvious difference between 

 individual fibres depends upon the nature 

 of the covering of the axon. When it 

 is coated on the outside by a more or 

 less thick sheath of a fatty substance, 

 termed myelin, it is said to be a myelinated 

 or medullated fibre. When the coating 

 of myelin is absent, the fibre is termed 

 a non-myelinated or a non- medullated 

 fibre. A second sheath thin, delicate, 

 and membranous, and placed externally 

 may also be present in both cases. It 

 is termed the primitive sheath or the 

 neurolemma. From a structural point of 



Axon 



4 ^ _ NBRVE . CELL FROM CEREBBLLUM (C ELL OK PO,* 

 SHOWING THE BRANCHING OF THE DENDRITIC PROCESSES 

 photograph by Professor Symington). 



