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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



depth of the muscle proper and terminate by 

 branching into numerous capillaries. 



There are impressive numbers of anastomoses be- 

 tween both arterial and venous vessels which form a 

 series of arcades. Direct anastomoses between ar- 

 terioles and venules are also found. 



The capillary bed of the perimysium is supplied by 

 metarterioles which come off at right angles from the 

 arterial or arteriolar arcades. These metarterioles 

 terminate as one or two capillaries which unite with 

 other capillaries to form venous effluent vessels. The 

 capillaries lie directly on the surface of the small 

 muscle bundles, thus each muscle bundle is sur- 

 rounded by a network of arterial and venous vessels 

 which interconnect freely with one another within 

 the connective tissue separating the bundles. The 

 muscle fibers are supplied with blood by branches 

 from the arteriolar arcades which penetrate the con- 

 nective tissue and give rise to capillaries which run 

 along the length of the muscle. 



Zweifach and Metz report the presence of metar- 

 terioles along the free margins of the skeletal muscle 

 which can be traced directly to the venous system. 

 These vessels, they believe, represent preferential 

 pathways which convey the most rapid stream of 

 blood from the arterial to the venous side. 



In addition to structural features, spontaneous 

 vasomotor changes were seen by Zweifach and Metz in 

 arterial and venous vessels. The vasoconstriction was 

 not often intense enough to stop blood flow through 

 the vessels involved, except at the level of the pre- 

 capillary sphincters. 



In investigations of red and white skeletal muscle 

 in rabbits (75, 113) injected preparations showed 

 arterial vessels which branched profusely to end in 

 capillaries running parallel to muscle fibers. Also 

 shown were numerous anastomotic connections be- 

 tween small vessels. 



Algire (4) and Algire & Merwin (5) studied the 

 panniculus carnosus through a transparent chamber 

 in the rat's back and also saw many arterial anasto- 

 moses as well as arteriovenous anastomoses. Arterial 

 branches from the subcutaneous layer supplied the 

 thin striated muscle layer with blood. The arterioles 

 from these branches subdivided into capillaries which 

 ran parallel to the muscle fibers, with cross connec- 

 tions between them, joining other capillaries to form 

 collecting venules. 



The capillary blood flow was noted to be inter- 

 mittent, the result of active vasomotion of the ar- 

 terioles. Algire & Merwin (5) estimated the length of 

 capillaries that were seen between the muscle fibers 



fig. 7. Capillary vessels in skeletal muscle. 



to be between 0.3 and 1.0 mm, with anastomoses oc- 

 curring at intervals of about o. 1 mm. 



Observations of the capillary network of the skeletal 

 muscle bands that course through the bat wing show a 

 vascular pattern similar to the descriptions given 

 above (see fig. 7). 



There are notable differences between the distribu- 

 tion of capillaries in the endomysium and that in the 

 areas adjacent to the skeletal muscle. The vessels 

 which run parallel to the muscle fibers are generally 

 longer and straighter than comparable vessels in the 

 surrounding connective tissue. An arteriolar branch 

 that crosses the muscle fibers often subdivides into 

 two capillaries that originate at right angles to the 

 parent vessel and go off in opposite directions. Thev 

 usually do not lie in the same plane, one going deep 

 between the fibers, occasionally until lost from view, 

 while the other vessel continues on the upper surface. 

 As a result of this downward, or sometimes upward, 

 turn it is possible to look directly down into the lumen 

 of a capillary rather than at the customary longitudi- 

 nal view. An arteriole with its accompanying venule 

 mav cross the muscle band without either of the vessels 



