MEDULLATED NERVE-FIBRES 457 



nerve-fibres. There are, however, many exceptions to this rule, as in 

 the case of movements following direct stimulation of the sympathetic 

 ganglia and certain centres in the brain and spinal cord ; but the cells 

 of many of the ganglia belonging to the cerebro-spinal axis are insensi- 

 ble to direct stimulation and can receive only impressions conducted to 

 them by the nerves. 



The nerve-fibres act as conductors and are incapable of gener- 

 ating impulses. There is no exception to this rule, but there are dif- 

 ferences in the properties of certain fibres. The nerves generally, for 

 example, receive direct impressions, the motor filaments conducting 

 these to the muscles and the sensory filaments conveying the impres- 

 sions to the centres. These fibres also conduct impulses generated by 

 the nerve-centres ; but there are many fibres, such as those composing 

 the white matter of the encephalon and the spinal cord, that are insen- 

 sible to direct stimulation, while they convey to the centres impressions 

 conveyed to them by sensory nerves and- conduct to the motor nerves 

 impulses generated by nerve-cells. 



In the natural classification of nerve-fibres, they are divided into two 

 groups ; one embracing fibres that have the conducting element alone, 

 and the other presenting this anatomical element surrounded with cer- 

 tain accessory structures. In the course of the nerves, the simple fibres 

 are the exception and the other variety is the rule ; but as the nerves are 

 followed to their terminations in muscles or sensitive parts or are traced 

 to their origin in the nerve-centres, they lose one or another of their 

 coverings. These two varieties are designated as medullated and non- 

 medtillated fibres. 



Medullated Nerve-fibres. These fibres are so called because, in 

 addition to the axis-cylinder, or conducting element, they contain, 

 enclosed in a tubular sheath, a soft substance called medulla. This 

 substance is strongly refractive and gives to the nerves a peculiar 

 appearance under the microscope, from which they are sometimes called 

 dark-bordered nerve-fibres. As the whole substance of the fibre is en- 

 closed in a tubular membrane, these are frequently called nerve-tubes. 



If the nerves are examined while fresh and unchanged, their ana- 

 tomical elements appear in the form of simple fibres with strongly 

 accentuated borders. The diameter of these fibres is 2iro"o to TT^o ^ 

 an inch (10 to 15 JJL). In a short time the borders become darker and 

 the fibres assume a different appearance. By the use of certain 

 reagents, it can be demonstrated that a medullated nerve-fibre is com- 

 posed of three distinct portions : a homogeneous sheath, a semifluid 

 matter contained in the sheath and a delicate central band. 



The tubular sheath of the nerve-fibres, the neurilemma, is a some- 



