THE RESPIRATORY ORGANS. 175 



Because of their presence the lungs retract when the chest is 

 opened. 



When sections of the lung are examined under the microscope 

 it is difficult, at first, to identify the different portions, which are 

 cut in all directions. The smaller bronchi may be recognized 

 by the presence of cartilage in their walls. The bronchioles pos- 

 sess no cartilage, but are surrounded by a band of smooth mus- 

 cular tissue, the muscularis mucosae. This becomes thinner, then 

 incomplete, and finally disappears as the infundibula are reached. 

 The infundibulum, it will be remembered, is the space into which 

 the alveoli open. When seen in section it will appear as a round, 

 oval, or elongated space, according to the direction in which it 

 has been cut, bounded by scallops, each of which is the cavity of 

 an alveolus. In every section there will be many alveoli which 

 have been so cut that their openings into the infundibulum will not 

 be included in the section. These alveoli have a continuous wall 

 surrounding their cavities. Still other alveoli will have been cut 

 in such a way that a portion of their walls will lie in the plane of 

 the section and parallel to it, so that the flat surface of the alveolar 

 wall will be visible, surrounded by an oblique or cross-section, where 

 the wall meets the surface of the section. Those alveolar walls which 

 have been cut perpendicular to their surfaces will appear thinner 

 than those which have been cut obliquely. With these considera- 

 tions in his mind, the student can have little difficulty in identify- 

 ing the different portions of the section (see Figs. 147-150). 



