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 1-2000th and 1-4000th 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 fibrille; being of a 
yellowish-grey colour, they are sometimes designated t 
grey fibres. Their diameter averages between the 1-4000th 
and 1-6000th 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) 
butthere is no doubt that this is an incorrect notion, an 
that even the fibres of the ordinary nerve-trunks may prese 
the “gelatinous” aspect, probably from incompleteness 
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 
