PHARYNX AND LARYNX. 4 1 



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 oesophagus. 



This portion gives off side branches — the intercostal 

 arteries — to the ribs, during its course in the thorax. 



5. The Ductus Arteriosus may be found as a liga- 

 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. 



C. THE INTERNAL ANATOMY OF THE HEART. 



Cut across the great vessels about half an inch from 

 the heart, and remove the heart from the body. 



I. The Auricles. 



Cut away the outer wall of both auricles and wash 

 out the contained blood. 



a. The Right 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 fossa ovalis ; 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 



