2^5 NORMAL HISTOLOGY. 



occur along their course. On reaching the smaller and the terminal 

 ramifications of the bronchial tubes the nerves become broken into 

 fine non-medullated fibrillae, which pass to the muscular tissue of the 

 tubule as well as to the. mucous membrane. The exact mode of 

 final termination of the nerve-filaments within the pulmonary tissue 

 is still undetermined. 



THE PLEURA. 



The pleura resembles in structure other serous membranes, the 

 general characters of which have been already considered in Chapter 

 VIII. It consists of an endothelial covering, a connective- 

 tissue matrix, and subpleural tissue. The lining of the pleural 

 cavity is not of equal thickness in all parts, 

 FIG. 291. the visceral or pulmonary pleura being thin- 



nest as well as most firmly attached, while 

 the parietal or costal pleura is thickest, and, 

 owing to the well-developed subpleural tissue 

 existing in this region, less rigidly adherent. 

 The endothelium of the parietal portion 

 Section of human pleura cover- possesses ce \\ s more expanded and thinner 



ing surface of lung : a, endothe- *. . 



Hum; b, fibro-eiastic stroma ; m, than those covering the surface of the lung; 

 cut bundle of muscie-ceiis; /, the e i ements i n this latter position vary in 



peripheral layer of pulmonary ... . , , , *. _. . , ,, , _ 



t i SSU e. their size with the changes in the bulk of 



the pulmonary mass. Between the endo- 

 thelial plates minute stomata exist, which through the minute cana- 

 liculi indirectly communicate with the lymphatic spaces within the 

 subjacent tissue. 



The stroma of the pleura consists of fine bundles of fibrous 

 connective tissue intermingled with elastic fibres ; within the fibrous 

 lamellae the intercommunicating lymph- channels form a plexus of 

 considerable richness, which communicates on the one hand with 

 the pleural cavity through the stomata and intervening canaliculi, and 

 on the other with the neighboring lymphatics within the subpleural 

 tissue. 



The latter where developed as a layer of some thickness, as beneath 

 the parietal pleurae, is composed of loosely-disposed areolar tissue, 

 containing many elastic fibres. Upon the lung the subpleural layer 

 is intimately united with the pulmonary tissue, and forms a strong 

 superficial fibrous envelope, in which bundles of non-striped 

 muscle are also present. Within the stroma of the visceral pleura 

 the blood-vessels form a wide-meshed capillary reticulum over the 

 surface of the lung ; superficial vessels communicate with deeper 

 branches surrounding the interalveolar septa. 



The nerves of the pleura occur as infrequent stems, composed 



