9 i8 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



RADIATING ARTERIOLE 

 165 



fig. 20. Schematic drawing showing rela- 

 tions of basic vascular units exposed by the 

 fenestra and the average blood velocity in 

 micra per sec. [From Perlman & Kimura 

 (95)-] 



STRIA 

 VASCULARIS 



SPIRAL 



PROMINENCE 



SCALA 

 VESTIBULI 



COCHLEAR 

 DUCT 



SCALA 

 TYMPANI 



COLLECTING VEIN 



tion and secretion of perilymph and cndolymph is 

 still not clear, although it is believed that endolvmph 

 is secreted by the stria vascularis (40). It is interesting 

 that this fluid, which fills the scala media, differs in 

 ionic content from perilymph which fills the scala 

 vestibuli and the scala tympani. Unlike all other 

 extracellular fluids in the body, endolvmph is high in 

 potassium and low in sodium, thus more nearly 

 resembling an intracellular fluid. The very slow rate 

 of blood flow in the stria vascularis may be necessary 

 to allow the formation of this intracellular-like fluid. 

 It has been suggested, also, that the slow flow rate 

 and the absence of vasomotion in the strial vessels 

 contribute to the fact that blood flowing through the 

 cochlea is not heard. 



Preferential or Thoroughfare Channel 



The attempt to give a representative description of 

 vascular patterns in terminal vascular beds has not 

 escaped the usual criticisms aimed at generalization 

 from one animal to another or among different tissues 

 within the same animal. One of the major issues 

 among investigators of the microcirculation has been 

 the acceptance of a preferential channel as a com- 

 ponent of all capillary networks. A chronological 

 presentation of the development of the concept, the 

 modifications, and current status might assist in 

 clarifying the issue. 



The first description of the arteriovenous (a-v) 

 bridge, later to be called the thoroughfare or prefer- 

 ential channel, appeared in 1937 (143). Zweifach, 

 in studies of the mesentery, nictitating membrane, 



and undersurface of the tongue of the frog, described 

 two types of vessels present in these structures. One 

 was a continuous central trunk that connected an 

 arteriole and a venule. This vessel, a direct continua- 

 tion of an arteriole, was invested with widely sepa- 

 rated atypical smooth muscle cells which were less 

 responsive to mechanical stimulation than smooth 

 muscle cells of the arterioles. The a-v bridges did not 

 always take a direct linear course from the arterial 

 to the venous side, but appeared in three basic pat- 

 terns: /) a direct course without other terminal 

 branches, 2) a fountain-shaped pattern, 3) a horse- 

 shoe-shaped pattern, which was to become regarded 

 as the basic design for the preferential channel. The 

 a-v bridge was functionally different in that it 

 always had a patent lumen with uninterrupted blood 

 flow. The second type of vessel was the true capillary, 

 a nonmuscular vessel which was an off-shoot or 

 branch from the a-v bridge, not in the direct path of 

 blood flow from arteriole to venule. In a camera- 

 lucida drawing of the vascular pattern in the frog 

 mesentery it can be seen that the a-v bridge either 

 formed a loop to return to the venule accompanying 

 the arteriole from which it arose, or continued across 

 the capillary bed to join another venule (fig. 21). 



In a second paper the same year, the arteriovenous 

 bridge was reported in the mesentery and ear of the 

 mouse (150). In 1939, Zweifach (144) extended his 

 studies on living vessels in the mesentery, tongue, skin, 

 and intestinal wall of the frog, and in the mesentery 

 and ear of the mouse. Little new information was 

 added, but the functional significance of the a-v 

 bridge was stressed. The main central pathways were 



