510 ARTHUR WILLIAM MEYER 



of the longitudinal fibers are found grouped in bundles, 

 among the fibers of the much thicker circularis as is the ease 

 in the arteries as a rule. The fibers of the circularis which 

 are arranged rather loosely especially at the periphery, lie in 

 concentric layers, those in the peripheral layers being more 

 definite. No true adventitia can be said to exist although a 

 number of small blood vessels are occasionally found in the 

 small amount of cordal tissue that still surrounds the extra- 

 abdominal portion of the torn veins. 



The specimens of the intra-abdominal contracted portions 

 of the paired veins which were examined have a very similar 

 structure except that almost no circularis was present and that 

 the fibers are interlacing. But the unpaired portion contrasts 

 strongly with the former and the extra-abdominal portion. 

 The unpaired portion is usually only partly collapsed and not 

 contracted. Its walls are folded slightly and differ from the 

 more distal portions in the entire absence of the longitudinal 

 musculature, and in the occurrence of great variations in the 

 thickness of the circularis. Although composed of much more 

 closely apposed fibers the uncontracted circularis seems unusually 

 thin for the size of the vessel and is surrounded by a fairly definite 

 and vascular adventitia. In describing the umbilical vein in 

 the human fetus Herzog stated that the wall of the vein is 

 formed only by a well-developed media composed of fibrous con- 

 nective tissue in which non-striated muscle is distributed very 

 irregularly. Such an extremely irregular arrangement of the 

 muscle fibers was seen in the umbilical vein of a dog but two days 

 old and also in the hypogastric arteries of a newborn dog. In 

 the first specimen the muscle was so loosely and irregularly dis- 

 posed that it looked not unlike loose fibrous connective tissue 

 and the adventitia which contained a number of small para- 

 umbilical veins was thin and al3sent altogether in some places. 

 Interlacing muscular fibers were also found in both the paired 

 and unpaired portions of the vein of one lamb, however, and 

 this interlacing is not infrequently so extensive that no regular- 

 ity in the arrangement of the fibers of the media into circularis 

 and longitudinalis can be distinguished. The non-striated 



