490 ARTHUR WILLIAM MEYER 



and to a fibrous transformation i7i loco. Since, in contrast 

 with the vein the atrophic changes in the arteries take place very 

 gradually it is apparent that the condition in which the vessels 

 are found depends much upon the age of the animal. In old dogs, 

 for example, the degenerated and transformed arteries usually 

 cannot be seen near the apex of the bladder unless the latter 

 is distended and then only as very fine, fibrous, often more or 

 less discontinuous cords which gradually become somewhat 

 thicker as the base of the bladder is approached. As already 

 stated, instead of being attached to the apex of the bladder the 

 free ends of the retracted hypogastric arteries in the lamb usually 

 lie more or less curled up in a wide and loose fold of peritoneum 

 at the sides of the bladder just lateral to its apex. This position 

 is probably very largely due to the fact that they retract actively 

 immediately after birth and not simultaneously with the urachus. 

 Moreover, they lie in broad, loose peritoneal folds instead of 

 being intimately associated with the bladder and urachus and 

 with each other for ten or twelve days or more, before retraction 

 can occur and this later retraction is a very gradual and not a 

 sudden and extensive one. 



The fact that the arteries which retract earlier degenerate 

 much later than the vein, especially in the dog and sheep, has 

 already been mentioned and some of the factors involved have 

 been suggested. A further factor it seems to me lies in the fact 

 that after becoming attached to the urachus and the apex of 

 the bladder the arteries are alternately stretched and relaxed with 

 each successive distension and evacuation of the bladder. While 

 this stretching is a purely passive process the relaxation may 

 nevertheless be accompanied by a certain amount of active con- 

 traction of the vascular musculature the effect of which may be 

 retarded atrophy and degeneration. 



In rabbits twelve days old the hypogastric arteries and urachus 

 had already retracted completely and their free ends met at the 

 apex of the bladder while in a rabbit one year old they could not 

 be detected except in the region at the base of the bladder be- 

 cause they had atrophied so completely. 



