928 



HANDBOOK. OF PHYSIOLOGY 



CIRCULATION II 



(see fig. 25). Such changes have also been recorded in 

 rabbit ear veins (unpublished data). (See fig. 26.) 

 Recently, spontaneous changes in venous tone were 

 recorded from the arm veins of man (16). Folkow has 

 long supported the concept that rhythmic changes in 

 tone of vascular smooth muscle is due to myogenic 

 automaticity (42, 43, 45, 46). He points out (45) that 

 because the rhythmical reactions seem to be com- 

 pletely unsynchronized, even in closely adjacent 

 smooth muscle cells, it is improbable that they should 

 be due to activity in a local syncytial nerve cell plexus 

 in the vascular wall, as suggested by others (85). He is 

 of the opinion (43) that intravascular pressure in a 

 purely mechanical way to some degree will add "ex- 

 citatory drive" to myogenic activity as proposed by 

 Bayliss (10) and confirmed in Folkow's laboratory 

 (42). Further confirmation appears in recent studies of 

 forearm blood flow by Blair el al. (12). 



At the present time then, in concurrence with Lutz 

 & Fulton (81), the term "vasomotion" should refer 

 to anv active change in the diameter of a blood 



vessel. It may be seen in one form or another where 

 vascular smooth muscle exists, such as in arteries, 

 arterioles, terminal arterioles, precapillary sphincters, 

 venules, and veins. Any definite conclusions now as to 

 the actual mechanism or mechanisms which initiate 

 or control this vascular activity would be premature 

 insofar as both direct and indirect evidence indicate 

 that vasomotion in its various forms may be ac- 

 tivated or modified through the central nervous sys- 

 tem, reflexly or automatically, through myogenic 

 automaticity, or through local metabolic factors. 



This activity in venous vessels, especially if pri- 

 marily dependent on myogenic automaticity excited 

 by increased intravascular pressure, could serve as an 

 effective aid to venous return from postcapillary 

 vessels. On the arterial side it could serve as the regu- 

 lator of blood flow through capillary nets as well as a 

 protective mechanism whereby capillary vessels could 

 not be subjected to sudden or prolonged increases in 

 pressure which might rupture their thin walls. 



fig. 25. Spontaneous pressure waxes in a 

 small \ein in the hindleg of the dog. [From 

 Wiedeman (137)] 



rYVVY 



I Art. Press. 



10 Sec. 



TflJJjTrlT^^ 



fig. 26. Spontaneous pressure variations 

 in a small vein of the rabbit ear. 



Smoll Vein 



W^mfflTO^TOBfflffi 



10 Sec 



