284 ANATOMY IN A NUTSHELL. 



ligament and the posterior arch of the atlas, and its roof is made by the Com- 

 plexus muscle. The vertebral artery lies in a groove on the upper surface of 

 the posterior arch of the atlas in this triangle. The suboccipital nerve enters 

 this triangle between the vertebral artery and the arch of the atlas. The deep 

 cervical vein begins in this triangle. 



LESSON LXXIX. 

 Arteries. 



The pulmonary artery (Plate ( 'XXVIII) arises from the left side of the 

 base of the right ventricle in front of the aorta. It passes upward and back- 

 ward,beingtotheleftsideandin front of the ascending aorta. It is about two 

 inches long and ends under the transverse aorta where it divides into the right 

 pulmonary artery and the left pulmonary artery. This artery and the ascend- 

 ing aorta are enclosed in a common prolongation of the serous pericardium. 

 The right pulmonary artery, which is larger and longer than the left, passes 

 behind the ascending aorta and the superior vena cava where it pierces the 

 pericardium. It divides into the branches at the root of the lung, one branch 

 being for the upper lobe of the light lung and the other for the lower lobe. 

 The branch which passes to the middle lobe is derived from the upper branch. 



The left pulmonary artery passes in front of the descending aorta and the 

 left bronchus to end in the root of the left lung where it divides into an upper 

 branch for the upper lobe and a lower branch for the lower lobe. 



The remains of the ductus arteriosus passes from the left pulmonary artery 

 to the inferior surface of the transverse aorta. 



The aorta begins at the upper part e>f the left ventricle and passes upward 

 to the right, it then arches to the left and backward. It now elescends on the 

 lefl side of the vertebral column to pass through the aortic opening of the 

 Diaphragm and it ends opposite the' fourth lumbar vertebra where it elivides 

 into a right and left common iliac artery. The aorta has the following divi- 

 sions; 1 ' Ascending, (2) Transverse, (3) Descending, and this elescending por- 

 tion is divided into thoracic and abdominal. The part above the Diaphragm 

 is the thoracic aorta and the pari below the Diaphragm is the abdominal aorta. 

 The ascending portion commences behind the left half of the sternum at the 

 level of the lower border of the third costal cartilage. It passes obliquely up- 

 ward and forward to the righl where it ends at the upper boreler of the second 

 righl chondro-sternal articulation. It is about two inches in length. 



The Relations of the Ascending Aorta are: 



I.\ Front.— (1) Pulmonary artery, (2) Right auricular appendix. (3) 

 Pericardium, i 1 Righl pleura and lung, (5) Remains of thymus gland. 



Behind. — (1) Righl pulmonary artery. (2) Left auricle. 



Righi Side. (1) Superior vena cava, (2) Right auricle. 



Left Side.- Pulmonary artery. 



The Transverse aorta (arch) (Plate CXXVIII) is a continuation of the 

 ascending aorta from the upper bonier of the second right e'hondro-sternal 

 articulation. It arches to the left and backward with its e-onvexity upward 





