52 



ANATOMY. 



tral band is very important, as it is sometimes the only part of a 

 nerve-fibre left within the tubular structureless sheath, constituting 

 thus the so-called pale non-medullated nerve-fibre. This axis is also 



Fig. 28 a (The Author), portion of a spinal nerve giving off two branches, shown to be composed of a 

 number of parallel cords called funiculi (of which one is seen projecting), inclosed in a sheath. Magni- 

 fied slightly, b (Kolliker), white tubular nerve-fibres from different parts of the nervous system, in dif- 

 ferent states. Magnified 260 diameters. 1, five fibres of different sizes from nerves, three having the 

 double outline. 2, two fibres which have become varicose after removal. 3, two fibres, the fatty contents 

 of which have become altered by th action of water. 4, fibres from the brain : one large one, showing 

 the outer tube, the central axis or cylinder projecting at the upper end, and the intermediate white or 

 medullary substance. The four last fibres are remarkable for their fineness. 



the part which is said to be continued into the delicate offsets of the 

 branched nerve-cells, those processes being identical in structure with 

 the non-medullated nerve-fibres. As the medullary substance in the 

 tubular fibres forms a covering around the central band, it is spoken 

 of as the medullary sheath. These medullated, tubular nerve-fibres 

 compose the white parts of the brain and spinal cord, and the chief 

 substance of the various nerves ; but they also pass into and mix with 

 the gray substance of the brain, cord, and ganglia. They vary much 

 in size (see Fig. 28), being finest of all, 4, in the superficial layers of 



Pig, 29 



Fig. 29. .(a b, Kolliker ; c, d, Valentin.) a, origin of a spinal nerve from the spinal cord, by two bun- 

 dles of funiculi, or two roots, which join te form the trunk of the nerve. On one of the roots, the pos- 

 terior one in the body, is a small knot called a ganglion ; the other, or anterior root, is seen to pass over 

 the ganglion without entering it. ft, section plan of a ganglion, showing the fibres of the posterior root 

 passing amongst tire ganglionie corpuscles, and the fibres of the anterior root passing over them, c, four 

 separate ganglionic cells from a spinal ganglion, of different shapes, d, minute portion of a ganglion, 

 showing six corpuscles, three white tubular nerve-fibres, and a number of the gray nerve-fibres or connec- 

 tive tissue fibres with little dark nuclei, a, natural size ; b, magnified slightly ; c, d, about 100 diameters. 



the brain, fine in the nerves of special sense, and in the ganglia, 

 larger in the fore part of the spinal cord, and largest in the motor 

 nerves. 



