196 CIRCULATORY SYSTEM. 



and is principally expended within its lateral lobes. — Below the 

 origin of this vessel, from the sides of the arterial circle come 

 oft" three or four considerable branches, the principal of which 

 is the 



Posterior artery of the cerebrum. This runs ob- 

 liquely backward, across the crus cerebri and tractus opticus, and 

 afterwards mounts upon the posterior lobe of the cerebrum to 

 distribute its ramifications. 



The ciRCULUs arteriosus, then (as we have seen), is 

 formed, anteriorly, by the transverse branch of the anterior cerebral 

 divisions ; laterally, by the counnunicating arteries ; and pos- 

 teriorly, by the bij'urcalions of the basilar artertj. 



The Posterior Aorta, 

 Considerably longer and of greater volume than the anterior 

 aorta, is the main trunk from which aie derived the arteries of the 

 abdomen, pelvis, and posterior extremities, the posterior inter- 

 costals, and some fo-w of the thoracic arteries. It commences 

 opposite to the fourth dorsal vertebra, and is there some little 

 distance removed from the spine. From its origin, it makes a 

 curve, first upwaid and then backward — having the pulmonary 

 artery on its left, the termination of the windpipe on its right : 

 having reached the bodies of the dorsal vertebrae, it enters the 

 superior mediastinum, and afterwards directs its course straight 

 along the spine, inclined to the left side — having now the esopha- 

 gus and vena azygos to its right, the thoracic duct to its left. 

 The portion of the vessel within the cavity of the thorax, dis- 

 tinguished as the thoracic aorta, gives rise to many branches, 

 but they are but small. 



1. The bronchial springs from the under part of the curva- 

 ture of the trunk, bends its course downward and forward towards 

 the root of the left bronchial tube, at which place it divides into 

 two — the right and left bronchial arteries. These vessels pene- 

 trate their respective lungs in company with the right and left 

 bronchial tubes, to the branches of which they continue to cling 

 in the course of their ramification within the substance of the 

 parenchyma. 



2. The esophageal likewise springs from the concavity of 

 the arch, near to the former, in some instances before it; and 

 proceeds backward and to the right, to reach the esophagus. 

 Here it splits into a superior and an inferior division ; which both 

 course the entire length of the esophageal tube, distributing 

 branches right and left over its surface, and terminating at the 

 cardiac orifice in anastomosis with the gastric artery. 



«i. The 1 ntercosta H.S, the remaining branches, come o(I" in 



