378 THE PULMONARY VESSELS. 



ing to their average calibre (after injection), into six groups or orders, as shown in 

 the annexed table ; and this classification will be adopted in the following descrip- 

 tions, the order to which each principal artery belongs being indicated by Roman 

 figures within a parenthesis after its name. The average diameter of the pulmonary 

 artery and of the several parts of the aorta will be directly stated in millimeters, as 

 will also that of the innominate, subclavian, common iliac and external iliac 

 trunks, which are considerably larger than the arteries of the first order, but differ 

 too widely in size from one another to be included in one group. 



OBDEE. AVEEAGE CALIBEE. EXAMPLE. 



I. 8 mm. (| inch) Common carotid. 



II. 6 mm. (i inch) Brachial. 



III. 6 mm. (i. inch) TJlnar. 



IV. 3-5 mm. (l inch) Radial. 



V. 2 mm. (i inch) Posterior auricular. 



VI. 1 to '5 mm. (i to i inch) Supraorbital. 



The sanguiferous system consists of two great divisions, comprehended in the 

 lesser or pulmonic and the greater or systemic circulations. To the former belong 

 the pulmonary artery and veins, which will be first described. 



PULMONARY ARTERY AND VEINS. 



PULMONABY ARTERY. 



The pulmonary artery conveys the dark blood from the right side of the heart to 

 the lungs. The main trunk is a short wide vessel (diameter 30 mm.), which arises 

 from the summit of the inf undibulum of the right ventricle, and runs for a distance 

 of two inches or rather more backwards, as well as upwards, towards the concavity 

 of the aortic arch ; a little below the latter, and about on a level with the disc between 

 the fifth and sixth dorsal vertebrae, it divides into two branches the right and left 

 pulmonary arteries, which pass to the corresponding lungs. The mode of attach- 

 ment of the pulmonary trunk to the base of the ventricle is noticed in the description 

 of the heart. Contained entirely within the fibrous pericardium, it occupies a 

 position in relation to the anterior chest-wall immediately to the left of the sternum, 

 behind the anterior extremity of the second intercostal space, from which it is 

 separated by the left lung and pleura. At each side of its commencement is the 

 corresponding coronary artery springing from the aorta, and close to its sides are the 

 two auricular appendages. It is at first in front of the aorta, but as it passes back 

 it lies on the left side of that vessel, around which it describes a slight curve ; and 

 its termination is in a plane altogether behind the ascending aorta. The two vessels 

 are united together by connective tissue and by the serous layer of the pericardium, 

 which forms a single tube around them. Below and behind, the pulmonary trunk 

 rests upon the left auricle of the heart ; and on its left side (above the auricular 

 appendage) it is only separated by the pericardium from the pleura and lung. 



The right pulmonary artery (21 mm.), longer and somewhat larger than the 

 left, runs almost transversely below the arch of the aorta, and behind the ascending- 

 aorta and the superior vena cava, to the root of the right lung. It lies above the 

 left auricle of the heart, and crosses in front of the oesophagus. In the root of the 

 lung it* divides into two primary branches, the upper of which is distributed to the 

 upper lobe of the lung, and the lower, much the larger, to the middle and lower lobes. 



The left pulmonary artery (19 mm.), situated a little higher than the right, 

 passes nearly horizontally outwards and backwards, in front of the left bronchus and 

 the descending aorta, to the root of the left lung, and divides into two branches, of 

 which the lower is the larger, for the upper and lower lobes of the lung. From the 



