940 THE RESPIRATORY SYSTEM. 
present in the pulmonary substance as an accessory branch, which proceeds from the main stem 
as it traverses the lower 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 enters 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 which flows into the lung through the pulmonary artery is 
rendered bright red and arterial. 
Lobules of the Lung.—-A thin layer of subpleural connective tissue lies subjacent 
to the continuous coating which the lung receives from the visceral pleura. From the 
deep surface of this subpleural layer fine septal processes penetrate into the substance of 
the lung, and these, with the connective tissue which enters at the hilus 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 chi aracter 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 pyrifor m 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. 
Alveolar Ducts, Infundibula, and Air-cells.—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 ver vy acute angles, but as the finer ramifications 
are reached this character becomes much less apparent. There is no anastomosis between 
the bronchial branches. 
Within the various lobules the finer bronchioles send off further branches, which pro- 
ceed at right angles from them. Soon the ultimate tubes are reached. These are not 
cylindrical, but have their walls pouched out by numerous hemispherical diverticula. 
Such a bronchiole is called an alveolar duct, and the diverticula are the air-cells or alveoli. 
Finally the alveolar duct divides into two, three, or more terminal parts, which become 
expanded and form the club-shaped, blind terminations of the bronchial system of tubes. 
These cecal endings are the infundibula, and the walls of each are thickly covered by 
alveoli or air-cells, all of which open into the infundibulum as into a corridor. 
Structure of the Bronchi.—When the large bronchi enter the ling they become 
cylindrical, and lose the flattening on the posterior 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 division, 
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 pr esent in the form 
of incomplete rings, but in irregular plates or flakes deposited at various points around 
the wall. As the tubes diminish these cartilaginous deposits show a corresponding reduc- 
tion in size, until at last, in bronchi of 1 mm. in diameter, they disappear altogether. The 
glands in relation to the tubes for the most part cease to exist about the same point. 
The muscular or middle coat, which in the trachea and primary bronchi is confined to 
the back wall of the tube, forms a continuous layer of circularly-arranged bundles in the 
bronchi as they ramify within the lung. Spasmodic contraction of the muscular coat gives 
rise to the serious symptoms which accompany asthmatic affections. The muscular fibres 
of the middle coat may be traced as far as the infundibula, on the walls of which they are 
present in considerable numbers. The mucous lining of the tubes becomes greatly thinned 
as it is followed into the smaller bronchioles. It contains a large number of longitudinally- 
arranged elastic fibres, and is disposed in longitudinal folds, so that when the tube is cut 
across the lumen presents a stellate appearance. The mucous membrane is lined by 
ciliated columnar epithelium. 
Structure of the Infundibula and Alveoli.—The walls of the infundibula and 
alveoli are exceedingly fine and delicate, but nevertheless constituents continuous with - 
those observed in the three coats of a bronchus are found entering into their construction. 
The epithelium is reduced to a single layer. Further, it is no longer columnar and 
