SPIRAL VESSELS. 
95 
distinction is, the size of the 
fibre in the vegetable being 
larger than that in the animal. 
In both, the fibre is within the 
membranous tube. In Fig. 83, 
the tracheae are displayed rami- 
fying on the coats of the 
stomach of a Bee , I may here 
remark that the tracheae serve 
the purpose of carrying oxygen 
to the blood or nutritious fluid, which in insects is 
distributed through the system generally, and not in 
vessels. 
Spiral vessels are elongated cells, acuminated at each 
end (Fig. 86, a) with one or more fibres developed within 
them, and capable of unrolling by the tearing of the 
delicate membrane of the cell-walls. I have already 
described the mode of formation of the spiral fibre by 
a secondary deposit in a spiral direction, on the internal 
surface of the cell- wall. The spiral vessel is subject to 
many modifications, in consequence of the variable 
arrangement of the secondary deposit, and these modifi- 
cations have heretofore been classed as distinct orders of 
vessels — the term spiral vessel having been restricted to 
such as contain one or more spiral fibres, and are capa- 
ble of being unrolled. Recent and accurate researches 
have demonstrated, that all the forms of vascular tissue 
are referable to the same origin : the simple cell with 
its secondary interrupted deposit. Wc have already 
FIG. 83 . 
Trachese ramifying upon the 
stomach of a Bee. 
