STRUCTUEE OF THE NEKVOUS TISSUE. 507 



two groups ; one embracing those fibres which have the conducting element 

 alone, and the other presenting this anatomical element surrounded by 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-medullated fibres. 



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

 to the axis-cylinder, or conducting element, they contain, enclosed in a tubu- 

 lar sheath, a soft substance called medulla. This substance is strongly re- 

 fractive and gives 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 enclosed in a tubular membrane, these are fre- 

 quently called nerve-tubes. 



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

 tomical elements appear in the form of simple fibres with strongly accentu- 

 ated borders. The diameter of these fibres is ^Vrr ^ n*oo ^ an i ncn (10 to 

 15 p). In a very short time the borders become darker and the fibres assume 

 an entirely different appearance. By the use of certain reagents, it can be 

 demonstrated that a medullated nerve-fibre is composed of three distinct 

 portions ; viz., a homogeneous sheath, a semi-fluid matter contained in the 

 sheath, and a delicate, central band. 



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

 elastic, homogeneous membrane, never striated or fibrillated, and generally 

 presenting oval nuclei with their long diameter in the direction of the tube. 

 This is sometimes called the sheath of Schwann. In its chemical and gen- 

 eral properties this membrane resembles the sarcolemma, although it is less 

 elastic and resisting. It exists in all the medullated nerve-fibres, large and 

 small, except those in the white portions of the encephalon and spinal cord. 

 It is not certain that it does not exist in the small, non-medullated fibres, 

 although its presence here has never been satisfactorily demonstrated. 



The medullary substance fills the tube and surrounds the central band. 

 This is called by various names, as myeline, white substance of Schwann, 

 medullary sheath, nervous medulla etc. It does not exist either at the ori- 

 gin of the nerves in the gray substance of the nerve-centres or at the periph- 

 eral termination of the nerves, and it is probably not an essential conducting 

 element. When the nerves are perfectly fresh, this substance is transparent, 

 homogeneous, and strongly refracting, like oil ; but as the nerves become 

 altered by desiccation, the action of water, acetic acid and various other 

 reagents, it coagulates into an opaque, granular mass. In the white sub- 

 stance of the encephalon and spinal cord, the neurilemma is wanting and 

 the fibres present only the axis-cylinder surrounded with the white substance 

 of Schwann. As a post-mortem condition, these fibres present, under the 

 microscope, varicosities at irregular intervals, which give them a peculiar 

 and characteristic appearance. 



The medullated nerve-fibres do not have regular outlines, but present con- 



