CHANGES IN VASCILAR PATTERNS 



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FIG. 4. A series of four arteriovenous communications from 

 the rabbit's ear. The curves represent the rapid and uncoordin- 

 ated rate of contraction or dilatation of each of these structures. 

 [From Clark & Clark (34) as modified by Clara (29).] 



an external layer of circular muscle in some instances, 

 the accumulated epithelioid cells simply acting as a 

 cushion that partly restricts the lumen even when the 

 intercalated segment is open. In the absence of the 

 circular muscle however it has been thought that 

 closure is the result of swelling of the epithelioid cells, 

 in consequence of a still unknown process. Bcnning- 

 hoff (12) was the first to suggest this idea, and Havlicek 

 (71) called these cells "Quellzellen." Since these are 

 approximately spherical, shortening, as with ordinary 

 muscle fibers, is not possible. This mechanism has 

 been extensively considered by Mark (112, 1 13). 



Even direct observations have their limitations, 

 since the functions of these structures may be multiple, 

 and are not necessarily the same at all times, nor in 

 all vascular beds in the same animal, nor in different 

 species. There is now good evidence that they can re- 

 spond both to neural and chemical stimuli, but there 

 are numerous contradictions in details (119). It is of 

 interest that so large a glomeroid structure as the 

 coccygeal body can be removed, as in resection of the 

 coccyx, without known physiological effects (160). 



Local mechanical stimulation, such as rubbing, re- 

 sults in opening of the intercalated segments (34, 61). 

 The effects of temperature appear to be determined 

 in degree as well as direction by quantitative factors. 

 Upon warming the whole animal both Grant and the 

 Clarks found in rabbits a widening of the anastomoses. 

 Moderate cooling was accompanied by their closure, 

 and this was observed also in the paw of the dog by 

 Bostroem & Schoedel (24). Sonomoto (163) also 



found that the arteries and a majority of the arterio- 

 venous anastomoses were constricted in the rabbit's 

 ear during the winter. In the fingers of man, however, 

 the anastomoses were closed by warming to an ex- 

 ternal temperature of 33 to 37 C, while cooling pro- 

 duced the opposite result. In the rabbit's ear Grant 

 (61) found that cooling below 15 C produced an 

 opening of the anastomoses, and that with greater or 

 more prolonged reduction in temperature the arteries 

 also became dilated, whereupon there was a rapid 

 flow of blood through the anastomoses. He explained 

 that this phenomenon kept the extremities from 

 getting too cold. In human skin and in the feet of 

 birds Grant & Bland (62) further established this 

 function of the arteriovenous shunts by temperature 

 measurements. 



The consequences of anoxia were examined by 

 Schroeder et al. (158) with plethysmography methods, 

 under the assumptions that the volume of an ex- 

 tremity kept at an initial pressure of 35 mm Hg will 

 reflect changes in capillary pressure, and that of an 

 extremity compressed at 15 mm Hg will reflect 

 changes in venous pressure. They found that when the 

 dog was breathing an atmosphere containing 8 to 9 

 per cent 2 , the capillaries were wide without alter- 

 ation in the functional state of the arteriovenous 

 anastomoses, but when the oxygen concentration was 

 between 6 per cent and 8 per cent the anastomoses be- 

 came narrow without change in capillaries. In an 

 atmosphere of between 5 per cent and 6 per cent 2 , 

 perfusion was slowed in consequence of constriction of 

 the nutrient bed as well as of the shunts. 



Stimulation of the cervical sympathetic was noted 

 by Grant (61) to constrict the anastomoses as well as 

 the small arteries in the rabbit's ear. The denervation 

 of an extremity in the dog resulted in dilatation of the 

 shunts (24). Folkow (54) stated that the cutaneous 

 arteriovenous anastomoses become maximally dilated 

 as soon as their constrictor fibers are cut, provided 

 that there is no significant increase in hormone output 

 of the adrenal medulla. Claude Bernard's classical 

 observation that, when the peripheral end of the 

 chorda tympani is stimulated, the rate of blood flow 

 from the submaxillary vein becomes greater and the 

 blood becomes bright red, has been interpreted to 

 indicate the shunting of blood through the arterio- 

 venous anastomoses (71). After vagotomy Curri et al. 

 (40) reported that the arteriovenous anastomoses be- 

 came widely open but lost reactivity to various 

 stimuli. 



The injection studies of Vastarini-Cresi (179) had 

 suggested that, in general, vasoconstrictor substances 



