76 TUBULAR AND GELATINOUS NERVE-FIBRES. 



it is very liable to be altered by manipulation, a small excess 

 of pressure in one part forcing the contents of the tube 

 towards some other where they are more free to distend it, 

 and thus producing a swelling. The greater delicacy of 

 the tubular sheath in the fibrous substance of the brain 

 and spinal cord, renders its fibres peculiarly susceptible of 

 this kind of alteration, so that they often present under the 

 microscope a somewhat beaded appearance (fig. 22, B) ; when 

 carefully examined, however, without any previous disturb- 

 ance, these fibres are found to be as cylindrical as those of 

 the nerve-trunks. The diameter of the nerve-tubules is 

 usually between l-2000th and l-4000th of an inch ; but it 

 may be somewhat greater or considerably less than this 

 average. They are larger in the. nerve- trunks than they are 

 near their central termination in the brain ; and it is a remark- 

 able circumstance that the fibres of the nerves of " special 

 sense" are considerably smaller than the average in every 

 part of their course. The " gelatinous" fibres cannot be shown 

 to consist of the same variety of parts as the preceding ; for 

 neither the tubular sheath nor the white substance of 

 Schwann can be distinguished in them. They are flattened, 

 soft, and apparently homogeneous, sometimes showing a dis- 

 position to split into very delicate fibrillse ; being of a 

 yellowish-grey colour, they are sometimes designated the 

 grey fibres. Their diameter averages between the 1 -4000th 

 and l-6000th of an inch. As these "gelatinous" fibres 

 form a considerable proportion of the trunks of the Sympa- 

 thetic system of nerves ( 461), they have been supposed to 

 belong properly to it, and to minister exclusively to the 

 organic functions, like the non-striated muscular fibre ( 57); 

 but^there is no doubt that this is an incorrect notion, and 

 that even the fibres of the ordinary nerve-trunks may present 

 the " gelatinous " aspect, probably from incompleteness of 

 development. 



61. In the central organs of the Nervous system namely, 

 the brain and spinal cord of the Vertebrata, the ganglia or 

 knot-like swellings on the nervous cords which take their 

 place in the lower animals, and similar ganglia belonging to 

 the Sympathetic system we find a form of nervous tissue 

 altogether distinct from the preceding ; which, from its con- 

 sisting of large cells or vesicles, is generally known as the 



