STRUCTURE,] DUCTS MOHI/S VIEWS. 88 



rings, situated at considerable distances from each other. 

 An organisation entirely analogous is also found in the spiral 

 fibre, for there are spiral vessels traversed in the middle by a 

 narrow fissure, by which the decomposition of the simple 

 spiral fibre into two fibres placed at certain parallel distances 

 is indicated. 



" What chiefly militates against the formation of rings by 

 the united spiral coils of a spiral vessel, is the proportion 

 which the rings bear to the spiro'id fibres which unite them. 

 And first, when the organisation of the vessels is very regular, 

 the rings and the fibres are generally of the same width, which 

 could not be the case if the rings were composed of a double 

 twist of the fibre. If, then, the spiral fibres which unite the 

 rings are slender, the width of these fibres bears no exact 

 proportion to the width of the rings, and of the divisions 

 perceived in them; moreover, the fibres are sometimes sol- 

 dered to the rings, and sometimes separated from them. The 

 spiral fibres, when they are united to the rings, cannot be 

 considered in certain cases, and according to the form of the 

 point of union, as a part of the fibrous mass which forms the 

 ring; this part separating from the ring, and continuing in a 

 spiral direction. I have thought it right to explain these 

 considerations, in the first instance, upon the annular vessels 

 in a state of complete development, because observations 

 made on developed vessels are necessarily more precise and 

 certain than those made on young vessels ; not so much on 

 account of the larger size of the developed vessels; but because, 

 in consequence of the greater thickness of their fibres, of the 

 greater distance of these organs from each other, and of the 

 absence of the mucilage with which the young vessels are 

 gorged, these developed vessels present a much clearer con- 

 tour, and the organisation of their fibres is more easily 

 observed. Doubtless it is true that we ought not to infer 

 from the structure of a developed organ the mode of its 

 development ; but the examination of this structure is never- 

 theless of very great importance in studying the manner of 

 its development, since we always thence obtain the means of 

 proving the truth of any theory propounded on the history of 

 development, a theory which ought not to be in contradiction 



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