THE RESPIRATORY TRACT. 



261 



and alveoli have a flat pavement epithelium resting on an ap- 

 parently structureless basement-membrane. Outside of this are 

 numerous elastic and muscular fibres, curving around the cavi- 

 ties, and holding in their meshes the capillary blood-vessels. 

 The muscular fibres are very numerous in the walls, of the al- 

 veolar passages and infundibula. The alveoli have a diameter 

 of .1 to .4 mm., but their size varies greatly according to the 

 degree of inflation of the lungs. 



FIG. 111. Section through an infundibnlnm : or, entrance from the alveolar passage into the infundi- 

 bulum ; &, nuclei of smooth muscular cells. Magnified 30 diameters. F. E. Schulze. 



The epithelium in the alveoli of the fetal lung is columnar 

 in shape, so that a section of such a lung resembles a section 

 of a glandular organ. But when the alveoli are distended at 

 birth, the cells change their form. In transverse sections, either 

 real or optical, of the alveolar walls, the epithelial plates pro- 

 ject more or less into the cavity, according to the degree of dis- 

 tention of the lung. This change of shape undoubtedly occurs 

 during life with the alternating expansion and contraction of 

 the thorax, and should be taken into account in considering 

 the pathological changes of inflammation, collapse, etc. By 

 injection of a weak solution of silver nitrate (^ per cent.) into 

 the bronchi of a fresh lung, and its subsequent immersion in 



