28 
ORGANOGRAPHY. 
BOOK I. 
by a spiral fibre kept together as a tube by interlaced fibres. 
Again, the fibre itself has been by some thought to be a flat 
strap, by others a tube, and by a third class of observers a 
kind of gutter formed by a strap having its edges turned a 
little inwards. Finally, the mode in which they terminate, 
has been asserted to be a continuation of cellular tissue. 
With regard to the presence of an external membrane within 
which the spiral fibre is developed, an examination of it ex- 
ternally, by means of longitudinal sections of the surrounding 
parts, is scarcely sufficient to settle that point. The best 
mode of examination is to separate a vessel entire from the 
rest of the tissue, which may be done by boiling the subject, 
and then tearing it in pieces with the points of needles or any 
delicate sharp instrument : the real structure will then become 
much more apparent than if the vessel be viewed in con- 
nection with the surrounding tissue. From some beautiful 
preparations of this kind by Mr. Valentine and Mr. Griffith, 
it appears that the membrane is external : in the root of the 
Hyacinth, for example, the coils of the spiral vessel touch each 
other, except towards its extremities ; there they gradually 
separate, and it is then easy to see that the spiral fibre does 
not project beyond the membrane, but is bounded externally 
by the latter, which would not be the case if the membrane 
were internal : a representation of such a vessel is given at 
Plate II. fifj. 9. Another argument as to the membrane beinfj 
external may be taken from the manifest analogy that a spiral 
vessel bears to that form of cellular tissue (p. 16.), in which a 
spiral fibre is generated within a cellule : it is probable that 
the origin of the fibre is the same in both cases, and that its 
position with regard to the membrane is also the same. Sec- 
tions, moreover, may be obtained through the centre of spiral 
vessels, and then it is manifest that the fibre is internal, 
because it projects beyond the inside of the vessel, at every turn. 
It is more difficult to determine whether the fibre is solid, 
or tubular, or flat like a strap ; and Amici has even declared 
his belief that the question is not capable of solution with 
such optical instruments as are now in use. When magnified 
500 times in diameter, a fibre appears to be transparent in 
