NERVOUS TISSUE. 273 



branched processes called poles. A cell consists of 

 a granular protoplasm, sometimes slightly pigmented. 

 They contain a distinct, large, and clear nucleus which 

 contains one or two nucleoli. 



A cell-wall can at times be distinctly demonstrated 

 and probably always exists. In the cortex of the brain 

 the cells are sustained by a delicate, retiform, connective 

 tissue called the neuroglia. 



In the ganglia, which are collections of nerve-cells along 

 the course of the white fibres, in fact, diminutive centres, 

 the cells are sustained by the neuroglia and surrounded 

 by a distinct fibrous capsule. The white fibrous nerve- 

 tissue forms the greater part of the nervous system. It 

 consists of fibres which continue uninterruptedly from 

 the centre to their distribution. They neither inosculate 

 nor anastomose. Each fibre consists of a central con- 

 ducting fibre, the axis-cylinder ; an insulating material, 

 the white substance of Schwann; and an investing sheath, 

 the tubular membrane. 



The entire fibre is about T15 -V?F inch in diameter ; the 

 axis-cylinder, about -% -$-$ inch. The fibre and its com- 

 ponent parts vary, however, in size, both absolutely and 

 relatively. The tubular sheath is very delicate, homo- 

 geneous, and is formed by the condensation of the white 

 substance of Schwann. In addition to the complete 

 medullated fibre, consisting of axis-cylinder, white sub- 

 stance of Schwann, and tubular membrane, fibres are 

 found which consist only of the axis-cylinder and tubular 

 membrane or the axis-cylinder alone. The fibres of 

 Remak are naked axis-cylinders, much smaller than the 

 axis-cylinder as ordinarily met with. Such fibres are 

 found largely in the sympathetic system. The axis- 

 cylinder consists of very minute fibrillae. The white 

 substance of Schwann consists of albuminous and fatty 

 matter, 



H 



