27 
structure of the brain and nerves, &c. 
it makes an essential part. If the mass of the brain is kept in 
water for 48 hours, and a thin slice is cut from the medullary 
part of the cerebrum, and laid upon a glass plate previously 
wetted with water, and a drop T)f water is allowed to fall 
upon the slice, holding the glass a little obliquely, so that 
the water must run across the surface of the glass, the force 
with which it moves is sufficient to break down the medullary 
substance of the brain, so as to bring distinctly into view in- 
numerable loose globules, many fragments of fibres of single 
rows of globules, and bundles of fibres, some of them of con- 
siderable length, as represented in Pl. II. fig. 1. 
If the substance of the brain is laid upon a piece of dry 
glass, and the separation of its parts is attempted by instru- 
ments, it is impossible to effect it, as the viscous mucus ad- 
heres strongly to the glass, and the substance would be 
indistinctly daubed on the glass, in the manner of a pigment, 
a state in which the globules are not discernible. 
It is impossible to distinguish the fibres composed of glo- 
bules, in an opaque state of the substance ; for, although in 
the section of any part of the brain, by means of a very strong 
magnifying lens, lines are discernible, these lines are produced 
by the light and shade on the substance, and only denote the 
bundles of fibres of which the brain is composed, but not the 
simple globular fibres. 
The gelatinous mucus seems to dissolve readily, and mix 
with the Water ; and, being perfectly colourless and transpa- 
rent, is entirely invisible while the substance of the brain is 
fresh, or whilst it is immersed in water ; but if the water is 
left to evaporate, and the substance gets dry, the mucus 
collects round the loose globules and fibres in considerable 
