THE DEVELOPMENT OF THE RESPIRATORY SYSTEM. 



337 



stitutes a large part of the lungs, but as development proceeds, the more 

 rapid growth of the respiratory parts results in the relatively small amount of 

 connective tissue characteristic of the adult lung. 



Changes in the Lungs at Birth. At birth the lungs undergo rapid and 

 remarkable changes in consequence of their assuming the respiratory function. 

 These changes affect their size, form, position, texture, weight, etc., and 

 furnish probably the only certain means of distinguishing between a still-born 

 child and one that has breathed. In the foetus at term the lungs are small, 

 possess rather sharp margins and lie in the dorsal part of the pleural cavities. 



Lungs 



Pleural cavities 



Diaphragm 



FIG. 290. Transverse section of a pig embryo of 35 mm., showing the developing lungs (bronchial 

 rami surrounded by mesoderm). The oesophagus is seen between the two lungs; above the 

 oesophagus is the' aorta. The dark mass in the lower part of the figure is the liver. 

 Photograph. 



After respiration they enlarge, fill practically the entire pleural cavities and 

 naturally become more rounded at their margins. The introduction of air into 

 the air passages converts the compact, gland-like, foetal lung into a loose, 

 spongy tissue. The specific gravity is changed from 1.056 to 0.342. While 

 there is a gradual increase in the weight of the lungs during development, there 

 is a very sudden increase at birth when the blood is freely admitted to them 

 through the pulmonary arteries. The weight of the lungs relative to that of 

 the body changes from about i to 70 before birth, to about i to 35 or 40 after 

 birth. 



