500 ARTHUR WILLIAM MEYER 



surrounded and encroached upon by a wide band of fibrous 

 tissue in some of the large meshes of which isolated irregular 

 degenerating nuclei lie. However, as shown in figure 13 there 

 are no signs of endothelial proliferation or of infiltration and the 

 outer layers of the media are quite well-preserved. At the ex- 

 treme distal portion of the vessel all of the musculature is intact 

 and the lumen has its characteristic shape. Slightly farther 

 proximally it contains some blood, the intima is absent and 

 where the blood lies in contact with the wall the meshwork of 

 connective tissue extends out into it and contains newly formed 

 vessels (fig. 14). The rest of the lumen is bounded by the circu- 

 laris of the media for no elastica interna is present in this portion. 

 Where the blood which is not formed into a thrombus, fills the 

 whole lumen this proliferation of connective tissue is evident 

 over the entire circumference and in some places the lumen is 

 entirely pervaded by it. The relations of this connective tissue 

 to that in the media are so clear that one cannot doubt that it 

 is a direct continuation and remnant of that which was contained 

 between the muscle bundles of the media and that some of the 

 fenestra in the network previously contained muscle-filler bundles 

 which have degenerated and have been absorbed. 



That these degenerative and obliterative changes are not 

 invariable or constant in occurrence, however, is shown by a 

 specimen of a hypogastric artery taken from a dog about a year 

 and a half old as shown in figure 15. In this case all the con- 

 stituents of the wall of the vessel including the endothelium and 

 the elastica interna are still present and well-preserved although 

 the connective tissue is greatly increased in amount especially 

 at the periphery. The vessel is gradually becoming transformed 

 by the proliferation of the adventitial and inter-fascicular con- 

 nective tissue which is always present in comparatively large 

 quantities in vessels from mature fetuses. Farther centrally 

 (fig. 16) in the vessel — i.e., nearer its origin — ^there is but little 

 connective tissue near the lumen which is surrounded by poorly 

 preserved endothelium, a partial elastica interna and a very 

 well-preserved media which on one side is encroached upon 

 markedly and replaced by fibrous tissue. 



