210 AVILLIAM H. F. ADDISON AND HAROLD W. HOW 



is seen in the peripheral portions of the lobes, immediately under 

 the pleura, as if more of the liquid had found its way to the ends 

 of the lobules. 



The large mononuclear cells which were seen scattered about 

 in the fetal lung are now difficult to find, although they may 

 still be seen by searching. In comparing the prenatal and neo- 

 natal lungs of kittens and other litters of dog, this same condition 

 was noted. 



LUNG OF TWO-DAY-OLD DOG 



The structure of the lung at this stage resembles, in general, 

 the mature lung. Reference to figure 5 shows the texture to be 

 very open in character. By means of outline drawings on cross- 

 ruled millimeter paper the framework is found to be only 20 to 

 30 per cent of the entire area, the open spaces constituting the 

 remainder 70 to 80 per cent. As the lung enlarges with the 

 further growth of the animal, the spaces still further increase. 

 As it is generally expressed, the growth of the lungs does not 

 keep pace with the growth of the thorax, as a whole, and con- 

 sequently the framework of the lungs becomes more and more 

 stretched, and the spaces are thereby also enlarged. 



In the study of the lungs of these prenatal and postnatal ani- 

 mals other points are seen. The behavior of the lungs when the 

 thorax is opened is different in animals which have breathed, 

 and those which have not breathed. In the former the lung 

 retracts quickly into the dorsal parts of the thoracic cavity, while 

 in the latter little change takes place. The best method for 

 comparing the positions of the lungs within the chest cavity, in 

 these two conditions, is to inject the blood vessels with formalin 

 without opening the thorax. It is true that the formalin causes 

 a swelling of the tissues of the organs, but the relations are well- 

 preserved. When one opens the thorax under these conditions, 

 the fetal lung is found not to extend so far forward and not to 

 cover as much of the heart as the postnatal lung, but in gen- 

 eral the relations are very similar. The differences are the result 

 of the increase in size of the lungs due to inspiration, while the 

 size of the other organs remains the same. 



