THE LUNGS. 



159 



thoracic cavity compels the pulmonary alveoli to maintain a 

 certain expansion permanently. In consequence of their 

 great distensibility, the lungs follow the expansion of the 

 thorax. By means of their elastic power, and assisted by 

 the muscles of their canals, they contract at each expiration, 



Fig. 147. — Transverse section through the pulmonary substance of a child of nine months. A 

 number of pulmonary cells, b, surrounded by the elastic fibrous net-work, which bound them in a 

 trabecula-like manner, and, with the thin structureless membrane, forming their walls {11) ; d, por- 

 tions of the capillary net-work with their vessels curved in a tendril-like manner, projecting into 

 the cavities of the pulmonary cells ; c, remains of the epithelium. 



as far as the thoracic walls permit. It is only when the tho- 

 racic cavity is opened that the lungs with their alveoli com- 

 pletely collapse. 



The parietes of the pulmonary vesicles, a continuation of the 

 terminal canal system, is a very thin connective-tissue mem- 

 brane. It is surrounded by elastic fibres, finer and coarser, 

 sometimes single, sometimes aggregated in groups. The latter 

 are met with in the interalveolar septa. The fundus of the 

 pulmonary alveolus shows only the finest elements, measur- 

 ing 0.0011 mm., in part more isolated, in part connected in a 

 reticular manner. 



