CHAP. IL] RESPIRATION. 557 



lung, which in the upper part of the lung is reduced to a central 

 passage surrounded by the honeycomb work of the chambers. In 

 passing down from the upper to the lower part of the lung, we 

 find the septa become fewer, and the honeycomb more open ; the 

 tertiary septa soon fail, then the secondary, and at the very bottom 

 or end of the lung even the primary septa are absent. 



Each septum consists of a middle basis of connective tissue, 

 rich in elastic elements, provided with close-set networks of 

 capillaries and covered on each side with epithelium, the characters 

 of the epithelium and its relation to the capillaries being much 

 the same as in the newt. Hence in each septum the blood is 

 freely exposed to the air on each side of the septum ; and the 

 arrangement of the honeycomb work of the alveoli increases 

 largely the total surface exposed to the air, and so increases the 

 exposure of the blood. 



The plain muscular fibres present in the general wall of the 

 lung pass to a certain extent into the septa. As in the newt, at 

 the neck of the sac the peculiar flat 'respiratory' epithelium, for 

 now we may perhaps so call it, changes into ciliated epithelium ; 

 traces of ciliated epithelium are also present at the extreme ends 

 of the septa. 



317. Each of the lobes of which the mammalian lung is 

 made up may be seen, at times somewhat indistinctly, to be 

 divisible into lobules. The bronchia, or divisions of the right and 

 left bronchus respectively, dividing dichotomously, and running 

 between the lobules as interlobular bronchia, accompanied by 

 branches of the pulmonary artery and pulmonary veins, finally 

 plunge into and end in lobules as ' lobular ' bronchia. Within the 

 lobules the lobular bronchia divide in a more or less rectangular 

 manner into smaller ' intralobular ' bronchia or bronchioles, often 

 spoken of also as alveolar passages. Each such bronchiole ends in 

 an enlargement having more or less the form of an inverted cone, 

 called an infundibulum. Each infundibulum repeats to a certain 

 extent the structure of the whole lung of the frog, or rather is 

 intermediate between the lung of the frog and that of the newt. 

 The more or less conical chamber of the infundibulum narrowing 

 into its bronchiole is divided by a number of septa into secondary 

 chambers of a somewhat polygonal form, the septa being simple 

 and not as in the frog bearing secondary and tertiary septa. Each 

 of these secondary chambers is called an alveolus ; it has a base 

 which is part of the wall of the infundibulum, sides which are 

 formed by the septa, and a mouth which opens into the general 

 cavity of the infundibulum and so into the bronchiole. Similar 

 but less developed septa are projected into the more tubular 

 cavity of the bronchiole itself, dividing it, less completely, into 

 alveoli ; hence the name alveolar passage ; these wholly disappear 

 before the bronchiole on its way out from the lobule becomes a 

 definite bronchium. 



