1252 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



on the intact rabbit's ear have been supplemented by 

 a whole series of investigations by the Clarks (34, 36) 

 who used a chamber technique. Recently, some at- 

 tention has been paid to a possible secretory function 

 of these structures. 



Arteriovenous communications can be classified as: 



A. "Normal" arteriovenous connections 



1. Simple 



2. Complex 



B. -'Abnormal" 



1. Congenital 



a. Familial 



b. Isolated 



2. Progressive acquired 



a. Hemangiomatous 



b. Within neoplasms 



c. Associated with disease, e.g., cutaneous 

 spiders 



C. Traumatic 



D. Surgically induced 



NORMALLY OCCURRING ARTERIOVENOUS CONNECTIONS 



Structure 



Connections between arteries and veins can range 

 from simple bridges only slightly larger than capil- 

 laries (fig. 1) to complex channels with specialized 

 cells in their walls (figs. 2 and 3). The former have 

 been well described as components of the microcircu- 

 lation by Zweifach (200). Both extremes can be en- 

 countered, for example in the rabbit's ear (148). 

 Spanner, in particular, has emphasized the existence 

 of transitional forms (165). 



The more complex of these structures can be de- 

 rived from a larger artery at a bifurcation, one division 

 of which may be distributed to capillaries in the usual 

 fashion. The other, or both, can become remarkably 

 contorted and characteristically differentiated before 

 joining a vein. In the intermediate or intercalated 

 segment, called variously the Sucquet-Hoyer or 

 Hover-Grosser canal, the wall becomes thickened by 

 the presence of a broad layer of ''epithelioid" cells 

 which abut upon or partly replace the endothelium 

 and appear to be differentiated from smooth muscle 

 cells. At the beginning of the intercalated segment the 

 epithelioid and muscle cells may be intermingled. 

 When fully formed, however, the former approach a 

 spherical shape and contain few or no myofibrils. 

 Their cytoplasm is hyaline, or somewhat vacuolated, 

 and gives no reaction for glycogen, fat, or mucin. A 



fig. i. A relatively direct arteriovenous anastomosis from the 

 human ear. Specimen injected with Berlin blue, stained with 

 hematoxylin and cleared. The arrow points to the terminal 

 portion of the intercalated segment. There is a slight fusiform 

 thickening nearer the arterial end of the latter, suggesting 

 accumulation of muscle or epithelioid cells. [From Prichard & 

 Daniel (135).] 



circular layer of muscle fibers may or may not be 

 preserved externally to the epithelioid cells. The elastic 

 laminae usually disappear in the intermediate seg- 

 ment. The adventitia is a delicate collagenous re- 

 ticulum supporting a very rich plexus of both medul- 

 lated and nonmedullated nerves. The latter were well 

 described by Masson (117, 118), and also in some of 

 their finest details in the tongue of the dog by Brown 

 (25). The latter noted thin unmyelinated fibers to 

 terminate in the media and thick myelinated fibers 

 (afferents?) with termination in the adventitia (fig. 3). 

 Groups of such complicated arteriovenous anastomoses 

 may be closely associated to form a "glomus" which 

 may be enclosed within a dense connective tissue 

 capsule. 



Less complex arteriovenous anastomoses exist in 

 which the intercalated segment is not tortuous. In 

 some there is simply a well-developed inner layer of 

 longitudinal muscle fibers without special epithelioid 

 characteristics. 



Distribution and Size 



The distribution of arteriovenous shunts is now 

 known to be almost universal. Aside from the glomus 

 coccygeum, some of the largest and most complex 

 glomera in man have been described in the skin and 

 subcutaneous tissue on the flexor surfaces of the fingers 

 and toes and in the nail beds. Their numbers have 



