OF THE STURGEON. 
353 
is a structure and arrangement of parts so peculiar, that it 
cannot be described unless in successive steps. 
In the first place, then, from the posterior or vertebral 
margin of the duodenal orifice, proceeds a long band of 
animal matter down the entire length of the tube, and si- 
tuate as near as may be in its centre, so as to constitute a 
true axis to the intestinal cylinder. When the latter is cut 
open by a longitudinal incision through its whole extent, 
this longitudinal band, which may be justly named the 
aa;is, or central column of the intestine, appears to divide 
it into two halves ; or, to speak more precisely, its plane 
divides the intestine into two semicylinders. The duodenal 
orifice is rather to the right of the upper extremity of this 
pillar ; and it may be said to open in a sinus or hollow, si- 
tuate between this and a membrane to be noticed pre- 
sently. (Fig. S.) 
About one and a half or two inches from its upper ex- 
tremity, there extends from the right margin of the fleshy 
aa^is to the wall of the intestine, a thin membrane, which 
then winds obliquely round the former, and proceeds in 
this manner, attached, on the one hand, to the axis, and in 
the other, to the inner surface of the intestine the whole 
way down. The arrangement is similar to that of a cork- 
screw, with a rod of wood inserted into the cylinder of the 
worm to represent the axis, or like a spiral staircase, wind- 
ing round a central modiolus. The effect of this membra- 
nous slip winding round the axis is, that the tract of the in- 
testine, instead of being only the length of the tube, is 
greatly increased. The combination of parts now described, 
constitutes what is termed the spiral valve, — -an arrange- 
ment peculiar to fishes, and to the object and use of which 
we shall presently advert. Though long known, it has not 
been very distinctly described ; and I have therefore been 
VOL. VI. z 
