Prenatal Growth of the Human Bk>dy. 137 



Lungs. 



The lungs (Fig. 3, Tables IV and V) are relatively small at first. 

 They increase steadily in relative size, reaching at the maximum an 

 average of 3.29 per cent of the total body weight during the 4th 

 month. From this time on, they decline slowly (with considerable 

 variations) in relative size throughout the fetal period.^ 



In the full-term still-born (289 cases) the lungs averaged 1.71 

 per cent of the total body weight. In the live-born (202 cases) the 

 a-verage was 2.18 per cent, the difference doubtless being due chiefly 

 to the increased blood supply to the lungs when respiration begins. 



When respiration begins, the lungs expand to two or three times 

 their original volume. It is doubtful, however, whether there is any 

 postnatal increase in the relative weight of the lungs, excepting the 

 immediate increase when respiration begins. Vierordt gives 1.75 

 per cent of the total body weight for the lungs in the newborn (still- 

 born?) and 1.50 per cent for the adult. The adult lungs vary ex- 

 ceedingly in air and blood content, however, so that it is very diffi- 

 cult to determine their normal relative size (volume and weight). 



Asi to the comparison between right and left lungs, His (24) found 

 the anlage of the right lung larger than that of the left from the very 

 beginning, and attributed it to the asymmetry of the heart and mesen- 

 tery. In the youngest specimen observed by me (11 mm.), there was 

 no appreciable difference in size between the two lungs (Table IV). 

 Thereafter, however,, the right lung appeared constantly larger than 

 the left, averaging about 20 per cent larger throughout the fetal 

 period. In the newborn, the difference appeared somewhat larger, 

 being 25 per cent to 30 per cent. The ratio between the right and 

 the left lung is subject to considerable individual variation. It is, 

 however, apparently not correlated with any corresponding variation 

 in the size of the heart or thymus, but varies independently of these. 



In the adult, according to most of the text-books of anatomy, the 

 right lung averages only 10 per cent larger than the left. Data com- 



"Legou erroneously concluded that the lungs remain of about the same 

 relative size throughout prenatal life. The remarkably large relative size 

 of the lungs during the middle period of fetal life seems to haA'e escaped 

 all previous observers. 



