898 



and lymphatic glands, are placed on a plane posterior to the great blood- 

 vessels ; the pulmonary artery lies more forward than the bronchus, and to 

 a great extent conceals it, whilst the pulmonary veins are placed still farther 

 in advance. The pulmonary plexuses of nerves lie on the anterior and 

 posterior aspect of the root beneath the pleura, the posterior being the 

 larger of the two. 



The order of position of the great air-tube and pulmonary vessels from 

 above downwards differs on the two sides ; for whilst on the right side the 

 bronchus is highest and the pulmonary artery next, on the left, the air- 

 tube, in passing obliquely beneath the arch of the aorta, is depressed below 

 the level of the left pulmonary artery, which is the highest vessel. On both 

 sides the pulmonary veins are the lowest of the three sets of tubes. 



Before entering the substance of the lung, the bronchus divides into two 

 branches, an upper and a lower, one for each lobe. The lower branch is the 

 larger of the two, and on the right side gives off a third small branch which 

 enters the middle lobe of that lung. 



The pulmonary artery also divides, before penetrating the lung to which 

 it belongs, into two branches, of which the lower is the larger and supplies 

 the inferior lobe. The upper of these two branches gives the branch to the 

 middle lobe. A similar arrangement prevails in regard to the right pulmo- 

 nary veins, the upper one of which is formed by branches proceeding from 

 the superior and middle lobes of the right lung. 



STRUCTURE OF THE LUNGS. 



Coverings. Beneath the serous covering, already noticed, there is placed 

 a thin layer of subserous areolar membrane mixed with much elastic tissue. 

 It is continuous with the areolar tissue in the interior of the lung, and has 

 been described as a distinct coat under the name of the second or deeper 

 layer of the pleura. In the lungs of many animals, such as the lion, seal, 

 and leopard, this subserous layer forms a very strong membrane, composed 

 principally of elastic tissue. 



Lobules. The substance of the lung is composed of numerous small 

 lobules which are attached to the ramifications of the air-tubes, and are held 

 together by those tubes, by the blood-vessels, and by an interlobular areolar 

 tissue. These lobules are of various sizes, the smaller uniting into larger 

 ones ; they are bounded by flattened sides, and compactly fitted to each 

 other and to the larger air-tubes and vessels of the lungs, those on the 

 surface of the organ having bases, turned outwards, from half a line to a 

 line in diameter. Though mutually adherent by means of fine areolar 

 tissue, they are quite distinct one from the other, and may be readily sepa- 

 rated by disgection in the lungs of young animals, and in those of the 

 human foetus. They may be regarded as lungs in miniature, the same 

 elements entering into their composition as form the lung itself. The struc- 

 ture of a single lobule represents in fact that which is essential in the entire 

 organ, each lobule, besides its investment of areolar membrane, being made 

 up of the following constituents : the air-tubes and their terminating cells, 

 the pulmonary and bronchial blood-vessels, with lymphatics, nerves, and 

 interstitial areolar tissue. 



Air-tubes. The principal divisions of the bronchi, as they pass into the 

 lungs, divide into tubes of less calibre, and these again subdivide in suc- 

 cession into smaller and smaller bronchial tubes, or bronchia, which, diverg- 

 ing in all directions, never anastomose, but terminate separately in the 



