PHYSIOLOGY, AND PATHOLOGY OF THE CHTMPATS'ZEE. 



377 



The aortic arch (text-fig. 38, A. A) describes a small curve and 

 gives way to the descending aorta at the sixth dorsal vertebra. 

 Its relations are as in Man. It gives off the innominate and left 

 subclavian arteries from its convexity, and the main bronchial 

 artery to the left lung (L.B. A) arises from its concavity. The 

 innominate artery (LA), a large vessel about 2 cm. long, gives ofi' 

 the left common carotid (L.U.O.A) and divides behind the right 

 half of the manubrium sterni into the right co'tnmon carotid 

 (R.O.C.A) and right subclavian (R.S.A) arteries. The left 

 common carotid gives ofi" the thyroidea ima (T.I. A) close to its 

 origin. The intrathoracic parts of the left common carotid and 

 left siobclavian arteries are mostly as in Man, but the latter is 

 relatively larger and not so vertical. 



Text-figure 38. 



The main arteries of the thorax and pectoral extremity. Ax.A : axillary arterj' ; 

 O.A : occipital arterj' ; SU.A : subscapular arteries. Other letters in text. 



The pulmonary artery (text-fig. .38, P. A) is much more capa- 

 cious than in Man, and its left branch is united to the aortic 

 arch by a wide, open, ductus a.rteriosus (D.A). The presence of 

 the latter, and the origin of the left bronchial artery are of 

 interest from the embryological point of view, but the foramen 

 ovale is closed and the vente cavse are normal. 



The ductus arteriosus is the sixth embryonic arterial arch, and 

 the bronchial artery coming from the concavity of the aortic 

 arch possibly represents the remains of one of the vessels con- 

 necting the outer extremities of the embryonic arches. So we 

 have in this animal a combination of interesting embryological 

 conditions persisting. 



