R. J. BING AND K. D. HELLBERG 
811 
peaks in red cell velocity are seen :^ the first one 
immediately precedes or takes place at the on- 
set of ventricular systole ; the second is smaller 
and immediately follows ventricular systole. 
Therefore the first peak occurs at the end of 
atrial systole, the second during atrial diastole. 
Hemorrhage shifts those peaks, v^^ith the re- 
sult that the first occurs at the height of ventric- 
ular systole, the second during ventricular dias- 
tole. With nicotine, the control pattern is little 
altered, with the two peaks occurring at the on- 
set and at the terminaion of ventricular systole. 
SUMMARY 
By means of transillumination, we have been 
able to study certain features of the coronary 
microcirculation of the left atrium of the cat. 
We are aware of the limitations of the method, 
but we believe that transillumination at the 
present time is still the best means of obtaining 
clear pictures of the coronary microcirculation. 
Efforts are continuing to develop procedures 
for the visualization of the coronary micro- 
circulation by means of special methods.* 
The significant observation of the study is 
the presence of asymmetric capillary arrange- 
ment with countercurrents. This has consider- 
able importance for the oxygenation of the heart 
muscle. Recruitment and autoregulation may 
exist simultaneously in the same preparation. 
A definite pattern for red cell velocity in the 
capillaries of the left atrium was observed. Two 
major peaks in red cell velocity are seen: the 
first large one immediately precedes or takes 
place at the onset of ventricular systole; the 
second smaller one follows ventricular systole. 
Slight changes in phasic red cell velocity were 
seen after hemorrhage, nitroglycerin, and nic- 
otine. 
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