PATTERNS OF THE A-V PATHWAYS 



923 



changes in the local environment, and thus whether or 

 not blood flowed through capillary networks would be 

 determined by the immediate needs of the tissue. 

 Thus, no complex central nervous control is neces- 

 sary, if the postulate that terminal vasculature is 

 primarily under the control of local conditions is 

 acceptable. 



BLOOD FLOW THROUGH TERMINAL VASCULAR BEDS 



Capillary Contractility 



Ideas regarding contractility of capillary vessels 

 have come full circle, beginning and ending with the 

 concept that capillaries are noncontractile and the 

 blood flow through them depends on contraction or 

 dilatation of the arterioles which supply them. Dur- 

 ing the intervening periods, investigators have pro- 

 moted the concept of independent contractility of 

 capillary vessels, first believed to be brought about by 

 the contraction of perivascular or Rouget cells and 

 later thought to be due to the contraction of endothe- 

 lial cells. At present it is generally accepted that true 

 capillaries do not contract. By definition they are 

 devoid of muscular elements, so that muscular contrac- 

 tility is out of the question, and the endothelial cells of 

 which they are composed are also noncontractile. 

 The internal diameter of capillaries may vary, how- 

 ever, by passive response to changes in pressure or in 

 the size of the endothelial cells which form the basic 

 structure of their walls. 



Independent contractility of capillaries was a con- 

 troversial subject in the eighteenth century. The 

 opinion expressed by Haller (60) in 1 756 that capil- 

 laries did not contract was generally accepted by most 

 physiologists until early in the twentieth century [(54), 

 see also (87)]. At this time publications by August 

 Krogh (73) appeared. Krogh's belief that capillaries 

 really contracted is found in a description of an ex- 

 periment in his book "which demonstrates in a crucial 

 manner that the whole length of a capillary from an 

 arteriole to a venule can be contractile, that it cannot, 

 when contracted, be forced open by the available 

 arterial pressure. . . ." Krogh was convinced of the 

 independent contractility of capillaries but he also 

 believed that no evidence obtained thus far was con- 

 clusive enough to explain the mechanism by which 

 this was carried out. 



Two possible means of decreasing the diameter of 

 capillary vessels had been suggested. One was that 

 either osmosis, or imbibition by endothelial cells, was 



responsible, and the other was that active contraction 

 of extraendothelial cells occurred, as described by 

 Rouget (103) in 1873. Krogh believed that the imbibi- 

 tion theory was ruled out by data published by Stein- 

 ach & Kahn (119), showing that the outside diameter 

 of contracting vessels decreased, rather than remain- 

 ing constant or increasing as it would if the endothe- 

 lial cells enlarged. He believed that anatomical proof 

 was lacking to establish the functional role of Rouget's 

 cell. Because of this need for more histological infor- 

 mation, he encouraged Yimtrup to conduct a detailed 

 study of the structure of the capillary wall. Vimtrup 

 (123) examined stained sections of frog tongue and 

 found cells such as those described by Rouget. He 

 subsequently named them Rouget cells. He was also 

 successful in identifying these cells on living minute 

 vessels in the tail of newt larvae, and in seeing them 

 contract. The frequent spontaneous contractions and 

 dilatations of vessels seemed to occur at the location 

 of the nucleus of a Rouget cell. This was final proof 

 for Krogh that capillaries possessed independent con- 

 tractility, the contractile element being the Rouget 

 cell. He explained away the conclusions of the Clarks 

 (27, 28) that the Rouget cells were noncontractile by 

 saying that there was no proof that the cells they 

 described on vessels in tadpole tails were the same as 

 Rouget cells or that the contractions they saw were 

 similar to normal contractility. Krogh, convinced 

 that the controversy regarding capillary contractility 

 was settled, extended his belief in the Rouget cell to 

 include its occurrence on all capillaries in both 

 Amphibia and mammals. 



In a very short time, however, the concept of the 

 Rouget cell as a contractile cell controlling the diame- 

 ter of capillary vessels was challenged by detailed 

 studies on small vessels in the rabbit ear, a technique 

 introduced by Sandison (104) in 1924, and used by 

 him and the Clarks, in whose laboratory he began his 

 work. A chamber for the rabbit ear was perfected in 

 which original vessels as well as newly formed ones 

 could be watched for many months. In 1931, Sandi- 

 son (105) reported that the appearance of Rouget or 

 adventitial cells on newly formed vessels occurred in a 

 few hours. Using a magnification of 400 times he could 

 find a clear space between these cells and the vessel 

 wall or an endothelial nucleus. The cells did not re- 

 main fixed, but wandered along the vessel wall. 

 Sandison stated that the function of the Rouget cell 

 was obscure, except that it helped form a supporting 

 framework for the vessels. He was able to demonstrate 

 in the rabbit ear vessels that a widening of the space 

 between the adventitial cell and the arteriolar wall 



