8 9 6 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



migrate slowly along the vessel wall until it reached a 

 larger vessel where it was swept forward in the blood 

 stream. Plugging of small vessels by leukocytes was 

 found in normal fields with vigorous flow, and this 

 obstruction determined to some extent the flow of 

 blood through the capillary nets. 



In 1954, Webb & Nicoll (130) discussed the angle 

 which an arterial branch forms as it leaves its parent 

 vessel. The downstream angle was found to approach 

 45 degrees. Webb and Nicoll postulate that this helps 

 to insure almost equal pressure in the artery and the 

 branch which arises from it. A similar type of 

 branching is seen in the arcuate arrangement formed 

 by arterioles. Arterioles, however, usually leave ar- 

 teries at right angles. The arteriolar branches have 

 sphincters at their point of origin that regulate the 

 size of the lumen of the branching vessel as it leaves 

 the artery. 



The arcuate system formed by the arterioles affords 

 collateral pathways and contributes to a uniform 

 distribution of blood at uniform pressure within the 

 capillaries. 



The capillaries form an extensive anastomosing net 

 which is supplied by terminal arterioles arranged in 

 such a manner that no capillary net is very far away 

 from its arteriolar supply. 



Active vasomotion of arteriolar vessels is, according 

 to Webb and Nicoll, the principal factor of a local 

 nature that regulates blood flow and blood pressure in 

 the capillary beds. The activity of the muscular wall 

 of the arterioles, which constitutes active vasomotion, 

 is independent of central nervous control. Degenera- 

 tion of nerves supplying an area does not affect active 

 vasomotion in the smaller arterioles, nor does stimu- 

 lation of intact nerves, although this does result in a 

 contractile response from the arteries or the larger 

 arterioles. 



Blood flow and blood pressure in capillary nets, 

 then, are controlled by two factors, one being the 

 anatomical arrangement of the arterioles which form 

 arcades, and the second being the active vasomotion 

 of the arterioles which is determined by local condi- 

 tions. 



Further discussion of the arcuate patterns formed 

 by arterioles in the bat wing appeared in a report by 

 Nicoll & Webb (89) in 1955. Arteriolar vessels form 

 arcuate configurations. These anastomosing vessels 

 are approximately equal in diameter. Several distinct 

 arteriolar arcuate systems can be identified arising 

 from either an artery or a large arteriole. Two 

 characteristic features were found in the manner in 



* 



I 



I 



fig. 2. Enlargement of an arteriolar branch at its point of 

 origin. Bat wing. X875. 



which the arcuate systems began. One was the angle 

 of origin of the arteriolar vessels lrom the parent vessel, 

 and this was found to be 90 degrees or less in reference 

 to the forward direction of flow in the parent vessel. 

 The second characteristic feature is a dilatation or 

 enlargement of the arteriolar branch at its point of 

 origin compared to its diameter throughout its length. 

 Also, the inside diameter of the opening between the 

 parent vessel and branch is much smaller than the 

 average inside diameter of the branch (fig. 2). This 

 formation, described by Nicoll and Webb in the bat 

 wing and named "Indian Club," has not been de- 

 scribed in microscopic vessels in other terminal vascu- 

 lar beds. In view of the fact that the notable appear- 

 ance of the enlargement of a vessel at its junction 

 depends to some degree on tonus, it may not be 

 readily apparent in anesthetized animals in which 

 vessel tone is low. If the tonus of the branch is quite 

 low, there may be little or no apparent difference be- 



