434 Mr. Lawson Tait on the 



lumen of the tube. In the contracted artery it is different. The fibres 

 are seen distinctly to be arranged in bundles, and these bundles have 

 different directions. Some are seen to be cut across, whilst others can 

 be traced passing downwards through the section and ending on the 

 lower surface. When a very thin section is made, stained, dried on the 

 slide, and mounted in balsam or dammar, here and there two layers of 

 bundles are seen, one immediately above the other, crossing one another 

 like the limbs of St. Andrew's cross. Again, two sets of bundles may 

 seem to be arranged like the webs on the shaft of a feather ; longitudinal 

 sections show this still better. Prolonged exposure to the action of 

 acetic acid shows, in a longitudinal section of an artery, that the muscu- 

 lar fibres are arranged in bundles, the directions of which occupy a 

 double spiral (Plate 13. fig. 17), somewhat like that carefully described by 

 Dr. Bell Pettigrew as being the arrangement of the fibres of the heart. 

 Silver-staining also demonstrates this. I have made a diagram of this 

 arrangement (Plate 13. fig. 18), and it may be fairly well illustrated by the 

 way in which a ball of twine is cast on the spindle. If the hole through 

 which the spindle has been passed be supposed to represent the lumen of 

 the artery, a distention of that aperture would, if the string were elastic 

 as the fusiform fibres are, drag the string into a uniform direction con- 

 centric with the hole. There can be no doubt that such an arrangement, 

 especially in the external bundles, where it seems to be most marked, 

 would be of great mechanical advantage in the closure of the vessels. 



One makroscopic fact is certainly in support of this view, which is 

 that the length of an artery in the umbilical cord is very perceptibly 

 diminished after its distention. 



It remains a very interesting point to consider whether this spiral 

 arrangement will not be found to exist in other arteries as well as in 

 those of the umbilical cord. Dr. Beale figures it as occurring in the 

 longitudinal fibres of the aorta of the horse. 



In support of my statement that there is no endothelium in the 

 arteries of the cord I may detail the following observations. 



In the case of the vein silver-staining reveals no difference in the 

 arrangement of the inner surface before and after distention with air, 

 and the appearances are very much the same as is seen in the endothe- 

 lium of other vessels. But in the arteries, if the inner surface be silver- 

 stained before distention, the brown lines will be found to include long 

 fusiform spaces very different from those seen enclosing endothelium 

 elsewhere (Plate 14. fig. 25) ; and if the same artery be distended with air 

 and then stained, the fine brown lines will be found to be replaced by 

 thick and much less regular brown interspaces (Plate 14. fig. 26). There, 

 it appears to me, the fusiform cells are elongated to their utmost and 

 somewhat separated; and the appearances are inconsistent with my 

 experience of vessels elsewhere lined with endothelium, and they are 

 quite consistent with the appearances noted after the treatment of trans- 



