PATTERNS OF THE A-V PATHWAYS 



9°3 



fig. 8. Red blood cells "on edge" in a capillary of skeletal 

 muscle. 



giving off branches to contribute or to receive blood 

 from the underlying muscle. When capillary vessels 

 running parallel to the muscle fibers are confined to 

 the narrow space between two fibers, the capillaries 

 are flattened and the cells face the fibers (fig. 8), as 

 reported by Reynolds et al. (ioo) in similar vessels in 

 the myocardium. In a wider space, the cells are often 

 seen broadside. 



Anastomoses between the capillary vessels running 

 parallel to the muscle fibers are numerous, the con- 

 nections occurring sometimes between adjacent ves- 

 sels and just as frequently with vessels lying some dis- 

 tance away. Short connections between arterial and 

 venous pathways are also seen. 



Intermittent flow occurs in the capillaries of these 

 skeletal muscle bands as a result of spontaneous closure 

 of short duration of terminal arterioles that give rise 

 to the capillaries which lie in the endomysium. 



Studies made thus far in these beds have established 

 no characteristic pattern formed by vessels supplying 

 the muscle bands that deviates from what is normallv 



seen elsewhere, except for the parallel course of the 

 vessels lying between the muscle fibers. 



Microcirculation in Myocardium 



There is a paucity of descriptive literature on the 

 capillary beds in the myocardium. Although a few- 

 investigations on fixed material appear, no studies 

 have been made on circulation through the minute 

 vessels in the living animal, presumably because of 

 the difficulty of microscopic observations of an organ 

 in motion. 



In 1928, Wearn (127) studied sections of myocar- 

 dium obtained from man, cats, and rabbits. The ves- 

 sels were filled with material injected through coro- 

 nary arteries. Wearn observed that almost every 

 cardiac muscle fiber was in direct contact with one 

 capillary and some fibers were touched by two or 

 more. A muscle fiber was completely surrounded in 

 some instances due to numerous anastomoses between 

 capillaries. These interconnecting branches ran across 

 the parallel muscle fibers. Capillary vessels were 

 found to lie between the cardiac muscle fibers and 

 did not actually penetrate the muscle substance. 



Saunders & Knisely (107) reported having watched 

 through a microscope the circulation in the myocar- 

 dium of beating frog hearts. Blood flow was seen to 

 stop during systole and to flow profusely during 

 diastole. The cessation of flow in systole was brought 

 about by compression of the small vessels by the 

 contracted myocardial fibers. 



Reynolds et al. (100) studied fixed sections of heart 

 muscle, the hearts having been taken from dogs 

 without loss of blood from the coronary vessels. They 

 report that capillaries had a diameter of approxi- 

 mately 4 m- The capillaries were seen to run along the 

 muscle fibers as described by Wearn (127), about one 

 capillary to every muscle cell. The orientation of the 

 red blood cells within the capillary vessels was be- 

 lieved to be unusual, in that the cells were often 

 seen edgewise, i.e., with the flat surface of the red 

 cell facing the parallel myocardial fibers. The authors 

 conclude that the capillary' vessels running between 

 the muscle fibers are elliptical in cross section rather 

 than round. 



In normal hearts, more capillaries were found in 

 the epicardium than in the middle portion or the 

 endocardium. Various explanations, none conclusive, 

 were given for this. 



Terminal arterioles were identified by the presence 

 of an endothelium with distinguishable smooth muscle 

 cells along their walls. The terminal arteriole gave rise 



