PATTERNS OF THE A-V PATHWAYS 895 



if direct observations of the vessels controlling 

 peripheral resistance are employed. 



STRUCTURE OF TERMINAL VASCULAR BEDS 



From the foregoing section it is apparent that the 

 microscopic blood vessels which connect the venous 

 and arterial systems have been studied in a wide 

 variety of tissues with equal variety in the choice of 

 experimental animals. Differences in vascular pat- 

 terns and structural components are to be expected, 

 but these differences are minor compared to the more 

 general similarities among the various microcircula- 

 tory beds. It is this aspect that will be emphasized in 

 the following descriptions of the microcirculation. 



Microcirculation in the Bat Wing 



Utilization of the bat wing for studies of the struc- 

 ture and function of small blood vessels has a long 

 history. An interesting and detailed report appeared in 

 1852, written by T. Wharton Jones (68), who de- 

 scribed the impressive rhythmical vasomotion of the 

 veins. Scattered publications by other investigators 

 appeared (18, 63, 87) early in the twentieth century 

 when new interest in capillary circulation was at its 

 peak. The interest in the wing of the bat as a site for 

 microscopic observation of vascular beds was stimu- 

 lated in 1946, when Nicoll & Webb (88) published 

 the results of several years of observations. A descrip- 

 tion of the vessels and their patterns in the terminal 

 vascular beds of the wing follows. 



The major site of peripheral resistance was found 

 to be in the small arteries which anastomosed to make 

 interconnected channels or loops. These small arteries, 

 which arose from the main arterial plexus and formed 

 arteriolar nets, had the capacity for changing their 

 lumen size by vasoconstriction. The smaller arterioles 

 of the nets usually had an inside diameter that was 

 equal to or smaller than that of a red blood cell. 

 Nonmuscular capillaries arose as branches of any of 

 these vessels of the arteriolar plexus, the parent vessel 

 of the nonmuscular capillary being designated the 

 terminal arteriole. The muscular coat of the terminal 

 arteriole became less regular as the vessels advanced 

 peripherally, as did the number of muscle cells on the 

 branches arising from it. 



The pathways between the arteriolar and venous 

 plexuses were seen to be similar to those of the rabbit 

 ear, as described by Clark & Clark (34) and Sandison 

 (106), with no preferential channel to carry blood 



To venule 



fig. I. Paths of blood flow in capillary bed in small area of 

 the bat's wing. [From Nicoll & Webb (88).] 



from the arterial to the venous side. Blood was seen 

 to take alternate routes through the capillary nets. 

 At times, especially in the terminal arterioles, there 

 appeared to be a major path of flow through the 

 capillary vessels to the venules, but this path was seen 

 to be inconsistent and changed to alternate routes 

 with modifications in arteriolar or venular circulation 

 in adjacent regions (see fig. 1 ). 



Supravital staining made it possible to study the 

 arrangement of the vascular smooth muscle of the 

 various vessels. Arteries had both circular and longi- 

 tudinal muscle fibers, the latter disappearing in the 

 arteriolar vessels. The terminal arteriole gradually 

 lost its circular muscle investment until areas of bare 

 endothelium could be seen and finally a single coiled 

 muscle cell formed the precapillary sphincter. The 

 spiral arrangement of a muscle fiber continued for a 

 number of turns, presumably reaching a length of over 

 100 fj. if uncoiled. Postcapillary vessels acquired a 

 muscular coat in the region of the first valves, and thus 

 veins were formed. Veins had the usual double layer 

 of circular and longitudinal muscle fibers. 



Because of the small caliber of arterioles and capil- 

 laries, flow was frequently seen to stop due to obstruc- 

 tion by a leukocyte. In some instances, an internal 

 pressure increase would cause the leukocyte to move 

 on. At other times, the leukocyte could be seen to 



