I$8 FIFTEENTH LECTURE. 



In the lungs themselves the bronchi divide dichotomously 

 again and again, and thus become finer and finer passages. 

 The cartilaginous half rings disappear, simple lamellae appear- 

 ing in their place. Their last remains are still seen in canals 

 of 0.23 mm. The thin parietes have a simple ciliated epithe- 

 lium of 0.0135 mm - m height. Mucous glandules continue 

 far down, as also do the smooth muscles, which form regular 



rings round the bronchial ramifi- 

 cations, and possibly till near the 

 so-called pulmonary vesicles. 



At the end of the terminal bron- 

 chial branches (Fig. 146, a) we 

 now arrive at the true respiratory 

 portion of our organ. 



We have, first, thin-walled (0.4 

 to 0.2 mm. wide) canaliculi, the 



Fig. 146.— A portion of the lung of an a 1 v ~ n l ar mccTcr^c f^rli 11I7M 

 ape {cercof>itii,cus\ injected with quick- cUV CUldl pUbbd^Cb ^OCIlLUZe^. 

 silver; a, endxif a bronchial twig ; c, al- npi • i i ■/» ,• 



veoiar canal ; %, infundibuia. I heir acute-angled ramifications 



(c) are familiar. Communicating 

 with them laterally and also terminally are short, conical hol- 

 low structures (b), the primary pulmonary lobules or, as they 

 are commonly called, the infundibuia. 



As the gland lobule consists of the gland saccules or acini, 

 so does the just mentioned infundibulum consist of similar 

 structures, the pulmonary vesicles, pulmonary cells or alveoli. 

 They are less isolated from each other, however, and to a 

 certain extent present more diverticulations of their walls, 

 which meet in common cavities. At a later period, indeed, 

 there is not unfrequently an absorption of individual portions 

 of the walls. Such expansions of the wall of the alveolar 

 passage into pulmonary vesicles (c) are met with every- 

 where. 



On making a section through the lung tissue, we meet 

 with the alveoli in the form of rounded and oval spaces (Fig. 

 147, b, b). Their diameter varies from o. 1128 to 0.3760 mm., 

 and increases with the age. 



The hermetic enclosure of the respiratory organs in the 



