PHARYNX AND LARYNX. 4I 
and dorsally, forming the arch of the aorta, from 
which the arteries for the head, upper limbs, etc., are 
given off, and finally passes posteriorly along the 
dorsal wall of the thorax, the descending aorta, lying 
dorsal to the cesophagus. 
This portion gives off side branches—the zz¢ercostal 
arterves—to the ribs, during its course in the thorax. 
5. The Ductus Arteriosus may be found as a lhga- 
ment connecting the pulmonary artery, just before its 
division into right and left pulmonary arteries, with 
the aorta. In embryonic life it is a vessel forming a 
communication between these two vessels, and in 
adult life is occasionally found patent. It is a rem- 
nant of the connection between the fourth and fifth 
arterial arches of the embryo. 
Ce LAG ANTERNAL ANATOMY OF FARE PE ART. 
Cut across the great vessels about half an inch from 
the heart, and remove the heart from the body. 
7 bbe Auricles. 
Cut away the outer wall of both auricles and wash 
out the contatned blood. 
a. THE Ricut AuricLe. The wall of the auricle 
is thin; that of the appendix is thicker and marked 
internally by muscular ridges, as is also the wall of 
the auricle in part. 
b. THE SEpTuM AURICULARUM is the thin parti- 
tion between the two auricles. An oval depression 
in it is known as the /ossa ovalzs ,, it marks the posi- 
tion of the foetal foramen ovale. If the dog is young 
this may still be found open. 
c. THE Coronary Vein. The opening of this 
vein into the auricle is seen just posterior to the 
