STRUCTURE.] SPIRALS HAVE AN OUTER MEMBRANE. 69 



exact structure of spiral vessels. They have been considered 

 to be composed of a fibre only, twisted spirally, without any 

 connecting membrane ; or to have their coils connected by an 

 extremely thin membrane, which is destroyed when the vessel 

 unrolls ; or to consist of a fibre rolled round a membranous 

 cylinder ; or even, and this was Malpighi's idea, to be formed 

 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 consist in a gradual transition to cellular 

 tissue. 



With regard to the presence of an external membrane 

 within which the spiral fibre is developed, an examination of 

 it externally, by means of longitudinal sections of the sur- 

 rounding 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 was long since proved 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. fig. 9. Another argument as to 

 the membrane being external may be taken from the manifest 

 analogy that a spiral vessel bears to that form of cellular tissue 

 (page 52), 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. Longitudinal sections, moreover, of spiral 

 vessels may be obtained, and then it is manifest that the fibre 



