THE LUNGS. 



1S51 



The intralobular bronchus is accompanied by some areolar tissue, and certain 

 fibrous prolongations extend into the lobule from the connective tissue disposed 

 about its surface. Although superficially these appear to divide the lobule into from 



four to twelve parts, they 

 Fig. 1573. 



Bronchiole 

 \ 



Bronchiol 



Mucous, 

 membrane 



Cartilage 



Pulmonary 

 arterv 



Alveoli 



Section of lung, showing small air-tubes and branch of pulmonary artery. X 35- 



penetrate but a short dis- 

 tance. They are not real 

 partitions, and the sub- 

 divisions they suggest 

 have no morphological 

 significance. 



Structure. — As far 

 as their entrance into the 

 lungs, the bronchi pos- 

 sess essentially the same 

 structure as the trachea. 

 After the division of 

 the bronchus within the 

 lung, the cartilage-rings 

 are replaced by irregu- 

 lar angular plates, which 

 appear at longer and 

 longer intervals until 

 they finally cease, the last 

 nodules usually marking 

 the points of bifurcation 

 of the bronchi. Within 

 the walls of bronchioles 

 of a diameter of i mm. 

 or less cartilage is seldom present. As the cartilage disappears the unstriped muscle 

 broadens into a continuous layer, which, however, gradually becomes thinner as the 

 air-tube diminishes, and extends only as far as the terminal bronchi. Around the 

 circular openings, by which the latter communicate with the atria, the muscle is 

 arranged as a sphincter-like band 

 (Miller). F^°- ^574- 



The walls of bronchi of medium 

 size consist of three coats, which 

 from without in are : f i ) an exter- 

 nal fibro-elastic tunic which encloses 

 the cartilage and blends with the 

 surrounding lung-tissue ; (2) a usu- 

 ally incomplete layer of involuntary 

 muscle composed of circularly dis- 

 posed elements ; (3) the mucosa, 

 consisting of a stratum of compact 

 elastic fibres next the muscle, the 

 fibro-elastic stroma and the cili- 

 ated columnar epithelium. Mucous 

 glands, similar to those of the 

 trachea, are present, decreasing in 

 number and size until the bronchus 

 approaches i mm. in diameter, 

 when they disappear. Their chief 

 location is outside the muscular 

 layer, which is pierced by the ducts. 

 In addition to diffused cells within 

 the mucosa, more definite aggre- 

 gations of lymphoid tissue occur as minute lymph-nodules along the bronchi, the 

 points of bifurcation of the latter being their favorite seats. 



The epithelium lining the air-tubes retains the ciliated columnar type,, with many 



Tunica 

 propria 



Epithelium 

 Goblet-cell 



Muscle 



Fibrous tissue 

 Alveolar wall 



Cartilage 



Portion of wall of small bronchus. X i8o. 



