CHANGES IN VASCULAR PATTERNS 



1269 



a 



I k % 





m 



fic. 13. Adaptive changes in a seg- 

 ment of jugular vein which had been 

 inserted into the course of the carotid 

 artery for 86 days. At left is shown the 

 appearance of the vessel before its 

 exposure to the higher pressure. Key: 

 i — intima; m — media. [From Fischer and 

 Schmieden (51).] 



tension which, in general, appears to increase the 

 tone of muscle (9, 16, 171). This has been suggested 

 for the increased muscle characteristic of bullae in 

 pulmonary emphysema, the walls of which are under 

 stretch consequent to air trapping (106). A brilliant 

 experimental demonstration of this mechanism in 

 small vessels has been provided by Weibel (187). 

 In his experiment, increased tension in mesenteric 

 vessels was produced by stretching the mesentery 

 slightly and attaching it to the diaphragm. The inner 

 longitudinal muscle then did increase to a remarkable 

 degree. 



Gaps in the internal elastic lamina of the large 

 arteries serving as collaterals have been noted by 

 several observers (102, 108). Some newly formed 

 collaterals mav possess relatively little or no elastica 

 (16.). 



Newly formed collateral vessels that develop from 

 capillaries, as for example in the adhesions between 

 visceral and parietal pleura, ultimately acquire a 

 structure appropriate to their function as arteries 

 or veins at the size which they ultimately attain 

 (86). 



The growth of muscle in the walls of vessels func- 

 tioning as collaterals in the mesentery of the rat has 

 been well described by Weyrauch & De Garis (189). 

 They considered the stimulus to be increased blood 

 volume. The tortuosity of the vessels was said to be 

 the result of the fact that the muscle fibers do not 

 grow in a single plane and this may be one factor to 



account for the tortuosity of collaterals in general. 

 They also described the appearance of muscle in 

 vessels which they thought were newly formed. 



Less well understood than the structural changes 

 are the forces that bring them about. They are prob- 

 ably similar to those that govern the differentiation 

 of arteries and veins from the retiform capillary 

 anlagen of the early embryo, as previously discussed. 



Measurement of Collateral Circulation 



Attempts have been made to estimate the extent of 

 collateral circulation by both anatomical and physio- 

 logical methods. The former offers only a general 

 and not necessarily reliable guide to the latter, in the 

 sense that the size of a bridge cannot always provide 

 a clue to the magnitude, nor even to the direction of 

 traffic. 



The early observers, such as Morgagni and Porta, 

 made many excellent observations with the naked 

 eye. Direct visual observation continues to be of 

 value and details of the formation of smaller col- 

 lateral vessels can be observed microscopically at 

 intervals in the process of their formation, for example, 

 in the ear of the living mouse (127). John Hunter 

 (83) early used injection methods in his famous 

 studies of the new blood supply to the stag's antler. 

 Some of his casts of the vessels are preserved to this 

 day in the Museum of the Royal College of Surgeons 

 in London. Attempts have been made to quantitate 



