MEDULLATED XERVE-FIBRES. 19 



ments that present greater variations in size and appearance 

 than the nerve-fibres. Certain fibres found in the course of 

 the nerves between the muscles are as large as y^Vrr f an 

 inch, have dark borders, and possess three well-marked 

 structures ; viz., a tubular membrane, medullary contents, 

 and an axial band ; others, with the same structure, are only 

 saooo of an inch in diameter ; others have only the medul- 

 lary covering and the axial band ; and others present the 

 axial band alone. Most of these anatomical elements have 

 essentially the same physiological conducting properties; 

 the variations in their structure depending upon differences 

 in their anatomical relations. In view of these facts, it will 

 be convenient to adopt some anatomical classification of the 

 fibres. 



In the most simple classification of the nerve-fibres, they 

 are divided into two groups; one embracing those fibres 

 which have the conducting element alone, and the other 

 presenting this element surrounded by certain 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 sensi- 

 tive parts, or are traced to their origin in the nerve-centres, 

 we find that they lose one or another of their adventitious 

 elements. These two varieties we shall term : 1. The me- 

 dullated fibres, and 2. The simple, or non-medullated fibres. 



nUated Nerve-fbres. These fibres are so called by 

 French and German writers because, in addition to the axis- 

 cylinder, or conducting element, they contain, enclosed in a 

 tubular sheath, a soft substance called the medulla. This 

 substance is strongly refractive and gives the nerves a pecu- 

 liar appearance under the microscope, from which they are 

 sometimes called the dark-bordered nerve-fibres. . As the 

 whole substance of the fibre is enclosed in a tubular mem- 

 brane, these are frequently spoken of as nerve-tubes. 



If the nerves be examined while perfectly fresh and un- 



