PRENATAL AND NEONATAL LUNG 213 



* 



all much distended, but only a few of the respiratory spaces 

 have been affected. Those which have been distended, however, 

 are often o\'er-expanded, as may be seen by comparison with 

 figure 8, from a kitten of the same litter, born at normal time, 

 and which had breathed twelve hours. The remainder of the 

 lung is denser than the ordinary fetal lung, and has apparently 

 been compressed by the over-distention of a few of the spaces. 

 Evidently respiration was begun before the lungs were sufficiently 

 developed to assume their noraial function. A point of interest 

 seen in examining the sections is that the alveoli which are situ- 

 ated directly on the bronchioles are distinctly distended. No 

 doubt it was these alveoli which played a large part in aerating 

 the blood of the animal during the period that it survived, and 

 indeed it must be these which function first at the beginning of 

 normal respiration. 



We here wish to express our thanks to Professor Piersol for 

 helpful advice and criticism and to Mr. E. F. Faber for assist- 

 ance with certain of the drawings. 



SUMMARY 



1. During prenatal life the future respiratory passages are filled 

 with a liquid. With the first inspirations in air, the thoracic 

 cavity is enlarged by the action of the respiratory muscles, the 

 lung is thereby enlarged, and the Uquid in the trachea and 

 bronchi is drawn down into the lung, and is distributed along 

 the walls of the alveolar and other spaces. 



2. In microscopic sections of lung taken from fetal animals, 

 there is seen a finely granular substance (a precipitate from the 

 fluid present) widely scattered through the spaces; and large 

 mononuclear cells, probably phagocytic in function are found 

 uniformly distributed in small numbers. 



3. In sections of lung of neonatal animals the finely granular 

 substance is still found, usually close to the w^alls of the air- 

 spaces and fewer mononuclear cells are seen. 



4. Before breathing, the lining epithelial cells of the alveoli are 

 irregularly cuboidal with rounded nuclei; but after breathing, 

 with the increased area of the walls of the alveoli, the nuclei are 



