1098 THE KESPIEATOEY SYSTEM. 



however, has seriously affected the stability of this conclusion. Narath contends that the distinc- 

 tion between the eparterial bronchus of the right side and the hyparterial bronchi of both sides 

 is not one of fundamental importance, and that a branch which arises from the first 

 hyparterial bronchus on the left side and turns upwards into the apex of the left lung is the direct 

 equivalent of the eparterial bronchus of the right side. This he terms the apical bronchus, and 

 he believes that it represents the first dorsal branch of the left stem-bronchus. Huntington, in a 

 very convincing paper, strongly supports the contention of Narath, and holds that, except " for 

 purposes of topography, we should abandon the distinction between eparterial and hyparterial 

 bronchi." With Narath he regards the eparterial bronchus as a secondary branch which has 

 migrated in an upward direction on the main stem. According to Huntington, therefore, Aeby's 

 proposition should be amended as follows : 



Right side. Left side. 



Superior and middle lobes Superior lobe. 



Inferior and cardiac lobes Inferior lobe. 



The cardiac lobe mentioned in this table is the occasional azygos lobe to which reference has 

 already been made, and it is interesting to note that, whilst the lobe in question as a separate 

 entity is rarely seen in the human lung, the bronchus which corresponds to it is always 

 present in the pulmonary substance as an accessory branch, which proceeds from the main stem 

 as it traverses the inferior lobe of the right side. It receives the name of the cardiac bronchus. 



STRUCTURE OF THE LUNG. 



The lung is constructed so that the blood which reaches it through the pulmonary 

 artery is brought into the most intimate relation with the air which enters it through the 

 trachea and bronchi. An interchange of materials between the blood and the air is thus 

 rendered possible, and the object of respiration is attained. As a result of this inter- 

 change the dark, impure blood, wliich flows into the lung through the pulmonary artery, is 

 rendered bright red and arterial. 



Lobules of the Lung 1 . A thin layer of subpleural connective tissues lies subjacent 

 to the continuous coating which the lung receives from the pulmonary pleura. From the 

 deep surface of this subpleural layer fine septal processes penetrate into the substance of 

 the lung, and those, with the connective tissue which enters at the hilum upon the vessels 

 and bronchi, constitute a supporting framework for the organ. The lung is lobular, and 

 on the surface the small polygonal areas which represent the lobules are indicated by the 

 pigment present in the connective tissue septa which intervene between them. Although 

 no pigment is present, the lobular character of the lung is particularly well marked in the 

 foetus, and with a little care the surface lobules in the foetal lung can be separated from 

 each other by gently tearing through the intervening connective tissue. The lobules thus 

 isolated are piriform or pyramidal in form. The broad bases of these lobules abut against 

 the subpleural layer, whilst each of the deep narrow ends receives a minute division from 

 the bronchial system of tubes. The lobules which lie more deeply in the substance of 

 the organ are not so large, and are irregularly polygonal in form. 



The Lung Unit. The unit of lung-structure is the lung-lobule. This comprises 

 a terminal bronchus with its air-spaces, blood-vessels, lymph vessels, and nerves. 



The terminal bronchus of the lung-unit is attained as follows : The larger branches 

 of the bronchi, as they traverse the lung, give off numerous divisions, which, by repeated 

 branching, ultimately form a system of tubes which pervade the entire organ. At first 

 the bronchial divisions come off at very acute angles, but as the finer ramifications are 

 reached this character becomes much less apparent. The finer ramifications of the bronchi 

 are termed bronchioles, which by subdivision give rise to the respiratory bronchiole of the 

 lung-unit. 



Within the lung unit the respiratory bronchiole gives off a series of terminal bronchi 

 or alveolar ducts, each of which leads to a group of air-spaces termed atria. Each one of 

 the atria communicates, in its turn, with a further and secondary series of air-spaces 

 termed air-sacs or alveolar saccules, the walls of which are pouched out to form the very 

 numerous alveoli or air-cells of the lung-unit. 



Structure of the Bronchi. When the large bronchi enter the lung they become 

 cylindrical, and lose the flattening on the dorsal aspect which is characteristic of the 

 primary bronchi outside the lung. They possess the same coats as are present in the case 

 of the trachea and primary bronchi, but as the tubes become smaller by repeated divisions, 

 these coats become correspondingly thinner and finer. Certain marked differences also in 

 the manner in which the constituents of these coats are arranged become apparent. 



In the external fibro-cartilaginous coat the cartilage is no longer present in the form 

 of incomplete rings, but in irregular plates or flakes deposited at various points around 



