22 ORGANOGRAPHY. 



BOOK I. 



a fibre generated spirally in their inside, or having their walls 

 marked by transverse bars arranged in a spiral direction. 



Such appears to me to be the most accurate mode of de- 

 scribing this kind of tissue, upon the exact nature of which 

 anatomists are, however, much divided in opinion ; some be- 

 lieving that the fibre coheres independently of any membrane, 

 others doubting or denying the mode in which the vessels ter- 

 minate ; some describing the vessels as ramifying ; and a fourth 

 class ascribing to them pores and fissures, as we have already 

 seen has been done in cellular and woody tissue. It will 

 be most convenient to consider all these points separately, 

 along with the varieties into which vascular tissue passes. 



There are two principal kinds of vascular tissue ; viz. spiral 

 vessels (Plate II. fig. 9. 11.), and ducts (Plate II. fig. 13. 15, 

 16. 18. 20.) 



Spiral VESSELS (/y. 6, 7.) {Vasa sjnralia, Lat. ; Trachea oi 

 many ; FistulcB spirales of Malpighi ; Spiralgefcisse or Sclirau- 

 hengefdssc, Germ.;) are membranous tubes with conical extre- 

 mities ; their inside being occupied by a fibre twisted spirally, 

 and capable of unrolling with elasticity. To the eye they, wlien 

 at rest, look like a wire twisted round a cylinder that "is after- 

 wards removed. For the purpose of finding diem for examin- 

 ation, the stalk of a strawberry leaf, or a young shoot of the 

 Cornus alba (common dogwood) may be conveniently used; 

 in these they may be readily detected by gently pulling the 

 specimen asunder, when they unroll, and appear to the naked 

 eye like a fine cobweb. 



Very different opinions have been entertained as to the 

 exact structure of spiral vessels. They have been considered 

 to be composed of a fibre only, twisted spiraUy, 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 

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

 by a spiral fibre kept together as a tube by interlaced fibres. 

 Agam, 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 

 kuul of gutter formed by a strap having its edges turned a 

 httle mwards. Finally, the mode in v.hich they terminate. 



