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The Aorta and Peripheral Circulation. 
The aortic wall shows a similar structure, on a reduced scale, to 
that of the heart; the muscle fibres, however, are much finer and more 
sparsely distributed, and the extrinsic fibres are absent altogether. 
As already mentioned, the aorta runs forwards along the dorsal surface 
of the stomach and turns downwards through the notch which divides 
Fig. 4. Argas persiciis. Diagram of the heart and arterial circulation of the adult. 
X 50 diain. 
the antero-lateral lobes of the latter. On reaching the dorsal surface of 
the brain it opens into a large blood-sinus (see PI. XV; and PI. XVI, 
figs. 2 and 8, ao., s. jig .)—the 'perigcinglionic sinus —which appears to 
completely envelop the central nervous system. On either side of this 
