74 THE MECHANICAL FUNCTIONS. 



easily be drawn out and uncoiled, their elas- 

 ticity enabling them to retain their spiral shape. 

 The object of this structure appears to be that 

 of keeping the cavity of the tube always per- 

 vious, by presenting resistance to any external 

 force tending to compress and close it.* 



In many instances the inner fibres of the tube, 

 instead of forming a continuous spiral, appear in 

 the shape of rings, succeeding one another at 

 regular intervals, and constituting what are 

 called annular vessels (Fig. 15). They are gene- 

 rally larger than the spiral vessels. In other 

 cases, as was first observed by Hedwig, the 

 adjacent coils are found to be closely coherent 

 throughout the greatest part of their course ; 

 leaving, however, occasional intervals, where the 

 external membrane, being unprotected, appears, 

 from its transparency, as if spotted or perforated 

 in various places (Fig. 16). Every intermediate 

 stage may occasionally be seen in the transition 

 from one of these forms to the other, in conse- 

 quence of the various kinds of convolution, of 

 branchings, or of transverse junctions of fibres, as 

 well as the greater or less extent of their lateral 

 adhesions. All these varieties are met with, 

 not only in different vessels, but, as was observed 

 by Moldenhawer and Kieser, even in the dif- 



* Vessels are sometimes met with which appear to be formed 

 simply by the coils of a spiral fibre in close juxtaposition, and 

 unattached to any external envelope, or connecting membrane. 



