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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



fig. 3. An arteriovenous anastomosis 

 from the tongue of the dog. The richest 

 nerve supply is to the intercalated seg- 

 ment. The filaments end in all levels of 

 the wall, in which large rounded epi- 

 thelioid cells predominate. Portions of a 

 thick sensory fiber and sensory termi- 

 nations are seen in the adventitia. 

 [From Brown (25).] 



1 



cation of the principles of surface tension, that no 

 connections larger than 25 /* were present in the lungs, 

 intestines, or kidneys of rats and rabbits, in contrast 

 with the extremities of these animals. Fritts and co- 

 workers (55) stated on the basis of recently developed 

 methods utilizing simultaneous T-1824 and radio- 

 active krypton injections that if such shunts are 

 functional they could account for not more than 1 per 

 cent of total left cardiac output in normal human 

 subjects. These problems have been judiciously re- 

 viewed by de Burgh Daly (41, 42) with special 

 reference to possible influences of the nervous system. 

 Connections between bronchial and pulmonary 

 arteries are mentioned in the discussion of collateral 

 circulation in this chapter. 



Development and Fate 



Arteriovenous shunts, at least those with a differ- 

 entiated intercalated segment and complex "orga- 

 noid" structure, do not exist in embryos. Popoff (134) 

 could not find them in the extremities from 4.5 

 months of intrauterine life to term, although Clara 

 (30) stated that they may be present in the newborn. 

 After the age of 60 the complex cutaneous arterio- 

 venous shunts tend to undergo atrophy and sclerosis. 



Clark & Clark (36) observed the new formation of 

 arteriovenous anastomoses in transparent chambers of 

 the rabbit's ear where the tissue was induced to grow 

 into an originally vacant space. Here the anastomoses 

 were relatively straight, but were characterized by the 



addition of an extraendothelial layer of differentiated 

 cells. Stimuli leading to increase in blood flow seemed 

 to increase the formation of these structures. Most of 

 these shunts were temporary, and disappeared early 

 or late, but some were permanent and had the 

 property of contractility. This seemed to be associated 

 with the development of nerves. Newly formed arterio- 

 venous shunts were also found within 2 weeks after 

 resecting a marginal segment of the rabbit's ear (148). 



Function 



With the disclosure of arteriovenous anastomoses in 

 erectile tissue it became obvious that their functional 

 state must vary from time to time. The ability of these 

 structures to close was established in the living trans- 

 illuminated rabbit's ear as early as 1930 by Grant 

 (61 ). The use of the rabbit ear chambers with "pre- 

 formed tissue" provided a clearer view in the hands 

 of the Chirks (34), and they were able to make 

 quantitative observations on the number, size, and 

 rate of contraction of the anastomoses over intervals 

 of many months. The specialized intercalated seg- 

 ments with their greater thickness and complex mural 

 arrangements and rich nerve supplv showed a faster 

 and more complete contraction than the arteries, and 

 the rhythm was independent of that of the latter. No 

 explanation was apparent for the extremely variable 

 responses of various arteriovenous anastomoses (fig. 

 4). The mechanism of closure has come under dis- 

 cussion and has been considered to be contraction ol 



