334 OF THE VEIXS. 



course, as by those which we are pleased to consider normal in distnbu 

 tion ; and, secondly ', with regard to us, that \ve should ever be keenly alive 

 to what is passing beneath our observation, and ever ready in the most 

 serious operation to deviate from our course and avoid or give eyes to 

 our knife, that it may see, the concealed dangers which it is our pride 

 to be able to contend with and vanquish. 



PULMONARY ARTERY. 



The pulmonary artery arises from the left side of the base of the right 

 ventricle in front of the origin of the aorta, and ascends obliquely to the 

 under surface of the arch of the aorta, where it divides into the right and 

 left pulmonary arteries. In its course upwards and backwards it inclines 

 to the left side, crossing the commencement of the aorta, and is connected 

 to the under surface of the arch by a thick and impervious cord, the re- 

 mains of the ductus arteriosus. 



Relations. It is enclosed for one-half of its extent by the pericardium, 

 and receives the attachment of the fibrous portion of that membrane by its 

 upper portion. Behind, it rests against the ascending aorta ; on either 

 side is the appendix of the corresponding auricle with a coronary artery ; 

 and above, the cardiac ganglion and the remains of the ductus arteriosus. 



The Right pulmonary artery passes beneath the arch and behind the 

 ascending aorta, and in the root of the lungs divides into three branches 

 for the three lobes. 



The Left pulmonary artery ', rather larger than the right, passes in front 

 of the descending aorta, to the root of the left lung, to which it is distri- 

 buted. These arteries divide and subdivide in the structure of the lungs, 

 and terminate in capillary vessels which form a network around the air- 

 passages and cells, and become continuous with the radicles of the pul- 

 monary veins. 



Relations. In the root of the right lung, examined from above down- 

 wards, the pulmonary artery is situated between the bronchus and pulmo- 

 nary veins, the former being above, the latter below ; while in the left 

 lung the artery is the highest, next the bronchus, and then the veins. On 

 both sides, from before backwards, the artery is situated between the veins 

 and bronchi, the former being in front, and the latter behind. 



CHAPTER VII. 



OF THE VEINS. 



THE veins are the vessels which return the blood to the auricles of the 

 heart, after it has been circulated by the arteries through the various tis- 

 sues of the body. They are much thinner in structure than the arteries, 

 so that when emptied of their blood they become flattened and collapsed. 

 The veins of the systemic circulation convey the dark-coloured and im- 

 pure or venous blood from the capillary system to the right auricle of the 

 heart, and they are found after death to be more or less distended with 

 that fluid. The veins of the pulmonary circulation resemble the arteries 

 of the systemic circulation in containing during life the pure or arterial 



