STRUCTURE.] DUCTS MOHI/S VIEWS. 81 



superposed rings. When this latter division takes place,, it 

 generally occurs on every ring of a vessel. Frequently, 

 however, this does not occur; but divided and undivided 

 rings alternate in an irregular manner, the undivided rings 

 being sometimes of equal size, sometimes of half the size of 

 the divided rings, and sometimes of a size very inconsiderable 

 in comparison with the divided rings. The direction of this 

 line of division is parallel to the lateral edges of the ring, so 

 that, by this fissure, the ring is divided into two superposed 

 rings, which sometimes touch and sometimes are placed at a 

 little distance from each other. According to Schleiden, this 

 line of division proceeds from the coils of the spiral fibre 

 being more or less completely soldered together, and always 

 in pairs. We easily perceive that, in this case, the line of 

 partition, should be directed spirally from one edge of the 

 ring towards the other, and that it should not be parallel to 

 its edges, but as the latter is constantly the case, we must 

 reject this explanation of the origin of the line of partition. 



"J.n the developed annular vessel, the rings are either en- 

 tirely isolated, or two or three are joined together in different 

 ways. It not unfrequently happens that the line of partition 

 does not divide the ring throughout the whole of its circum- 

 ference, but that the two superposed rings are united for a 

 space variable in extent ; in which case the parts separted 

 are removed to a greater or less distance from each other, 

 and are placed obliquely to the axis of the vessel. In other 

 cases, and this is the habitual organisation, the rings are 

 removed to a greater or less distance from each other, and are 

 separated by a regular spiral fibre, which, according to the 

 distance of the rings, describes one or more volutions, and 

 frequently even a great number. Of this there are several 

 modifications : very generally from a ring will proceed a spiral 

 fibre of the same width as the annular fibre, the distance of 

 whose coils is nearly equal to that of the rings in the portion 

 of the vessel which exhibits this structure ; the other extre- 

 mity of the fibre being similarly annexed to a complete ring, 

 followed by rings either isolated or again reunited by spiral 

 fibres. 



VOL. I. ('. 



