PATTERNS OF THE A-V PATHWAYS 



899 



including arteriovenous anastomoses, appeared in 

 1934 (30). Clark and Clark again described the 

 fluctuation in rate and amount of blood flow through 

 any given vessel, as well as the frequent reversals in 

 the direction of flow. A single capillary or venule was 

 seen to have an abundant flow of blood in one direc- 

 tion and a few seconds later an equally great flow in 

 the opposite direction. The variation in flow included 

 scanty flow of a few blood cells, or plasma and plate- 

 lets only, or stasis, or complete emptying. Such 

 changes were brought about by periodic active con- 

 tractions of arterial vessels or portions of arterial 

 vessels. The numerous thick-walled arteriovenous 

 anastomoses were most conspicuous for their active 

 contractility. Their contractions were usually more 

 frequent, quicker, and more powerful than those of 

 the arteries, and their effect on venous circulation was 

 more sudden. Definite active contraction of veins was 

 reported to occur near the point of entrance of a 

 cluster of arteriovenous anastomoses. 



Clark & Clark (32) studied the growth of capillaries 

 into a transparent chamber and found that new 

 capillaries arose as endothelial outgrowths from vascu- 

 lar endothelium. They advanced as blindly ending 

 sprouts, connecting with neighboring sprouts to form 

 loops, and continued to advance as a plexus with a 

 growing edge of new sprouts. The growing vascular 

 network showed differentiation of vessels in the older 

 portions of the first-formed capillary plexus and many 

 of the capillaries were seen to retract and disappear. 

 An entire chamber was revascularized in 2 or 3 weeks 

 with further differentiation continuing through en- 

 largement of new arterioles which were receiving a 

 large blood supply and widening of venules draining 

 large amounts of blood. There was a further reduction 

 in surplus capillaries. After a few days, the vascular 

 pattern was relatively stable. 



The Clarks next directed their attention to the 

 development of extra-endothelial cells on the walls of 

 peripheral blood vessels (33). Three months after 

 vessels had regenerated it was found that venules were 

 wider than capillaries, both vessels having similar 

 walls, while arterioles were as narrow as capillaries 

 and narrower than venules. The walls of the arterioles 

 differed in number and arrangement, and in the 

 form of the extra-endothelial cells. Blood flow was 

 seen to be steady and rapid in arteries and arterioles, 

 steady and slower in veins, and slow with frequent 

 hesitations and reversals in capillaries. Circulation in 

 capillaries was variable, with intervals of steady flow- 

 being interspersed with periods of stasis, plasma 



skimming, or absence of flow during which the vessels 

 remained open and were filled with plasma. The 

 subsequent fate of the extra-endothelial cells depended 

 on the fate of the vessel on which they appeared. If 

 the vessel remained a capillary, they were occasionally 

 seen to increase in number by mitotic division or to 

 retain the same number. The cells were inert. If the 

 capillaries became parts of venules, the adventitial 

 cells increased in number, retained their longitudinal 

 arrangement, and remained inert. The change of a 

 capillary to an arteriole involved straightening of the 

 vessel, loss of side branches, narrowing of caliber, and 

 an increase in thickness of the endothelium. There 

 was a rapid increase in the number of extra-endothelial 

 cells which assumed a transverse position. Definite 

 active contractility was seen to develop in these cells 

 which became smooth muscle cells, providing they 

 were reached by a regenerating vasomotor nerve. 



The caliber changes in minute vessels were dis- 

 cussed by Clark & Clark (34) in 1943. In earlier 

 published studies, the attention of the authors had 

 been on the main arteries, their branches, the arterio- 

 venous anastomoses, and the larger veins. Observa- 

 tions on newly formed arteries indicated that the 

 number of arteries which developed contractility, the 

 rate at which contractility appeared, and its final 

 extent on individual vessels and their branches de- 

 pended on the rate and extent of growth of new vaso- 

 motor nerves. Terminal arterioles in original vascular 

 beds in the preformed type of chamber were seen to 

 show^ spontaneous contractions which in most cases 

 obliterated the lumen. These vessels could sometimes 

 be made to contract by prodding the animal, but 

 their behavior was erratic. A terminal arteriole was 

 seen to divide immediately beyond its last muscle cell 

 into a capillary plexus. In some instances a terminal 

 arteriole was prolonged for a distance beyond the 

 point of the final muscle cells before forming a capil- 

 lary plexus. Such vessels had longitudinally arranged 

 adventitial cells rather than muscle cells on their walls. 

 Except for this, they had the characteristics of ar- 

 terioles, being straight, uniformly narrow, and having 

 a relatively thick endothelium. The region of active 

 contraction was confined to the portion of the vessel 

 which had smooth muscle cells, but the distal portion 

 at times showed a narrowing, with protrusions of 

 endothelial nuclei into the lumen, after blood flow 

 was shut off by active contraction of the proximal 

 portion of the vessel. The vessel showed an increase in 

 caliber following increased blood flow through it. 

 The Clarks refer to these vessels as arterial capillaries. 



