90 ZOOLOGY. 



nerve-fibre (see below). Bipolar cells appear to have no 

 arborizing processes, but send off two axis-cylinders ; while 

 unipolar cells are simply bipolar ones in which the two axis- 

 cylinders are united at the base (fig. 51, B). 



8. Nerve-fibres are really complex structures, con- 

 sisting of an axis-cylinder (which is a part of a nerve-cell) 

 and a connective-tissue covering called the neurilemma. 

 When only these two structures are present the fibre is said 

 to be non-medullated. More usually there is between these 

 two parts a white layer consisting chiefly of fatty material : 

 this is called the medullary sheath, and it is always inter- 

 rupted at intervals, called the nodes of Ranvier (see fig. 51). 

 Such a fibre is a medullated fibre. As a rough rule we may 

 say that non-medullated fibres are associated with unstriated 

 muscle-fibres, and medullated with striated fibres and with 

 most of the sense-organs. Nuclei are found at intervals 

 along the course of all nerve-fibres, and these were formerly 

 supposed to be the nuclei of cells which had united longi- 

 tudinally to form the axis-cylinder. This is now known 

 to be a mistake the nuclei belong to the cells of the 

 neurilemma ; the only nucleus that can be said to belong 

 to the axis-cylinder is that of the nerve-cell from which it 

 is derived. 



9. Functions of Nerve-fibres. Nerve-fibres are often 

 compared to telegraph-wires, the nerve-cells answering to 

 the telegraphic instruments in an office. The comparison 

 is a useful means of realizing the working of the nervous 

 system, but it cannot be carried into detail. For one thing, 

 a nerve-fibre can only convey a message in one direction 

 usually, but not always, from the nerve- cell to its outward 

 termination. Nerve-fibres can be classed on this ground as 

 afferent (conveying messages towards the central nervous 

 system), efferent (conveying them away from ifc) and intra- 

 central (conveying them from one part of the central system 

 to another). Afferent and efferent fibres as they extend 

 beyond the central system are bound together by connective 

 tissue into bundles, and these into larger bundles or nerves. 

 A nerve may consist entirely of afferent fibres, when it is 



