206 WILLIAM H. F. ADDISON AND HAROLD \V. HOW 



of the present-day text-books of physiology and pathology. On 

 the other hand, it is very easy to increase the size of the spaces 

 artificially. In J. AI. Flint's paper on the development of the 

 lungs ('06), he has an illustration of the lung of a fetal pig 27 

 cm. long, which is a stage shortly before birth. For the pur- 

 pose of preservation the lung was injected intra-tracheally with 

 the fixing fluid and this fact is responsible for the appearance of 

 the lung in section. The appearance resembles very closely that 

 of a lung which has breathed, as one can see by comparing it 

 with his next figure, no. 29, that of a two-day-old pig. This is 

 because the spaces were distended by the injection of the fixing 

 fluid and indeed, as we found with sheep fetuses, relatively little 

 force is required to distend the fetal lungs when introducing fluid 

 through the trachea. In consequence of this distention the lin- 

 ing cells of the respiratory lobules are artificially stretched and 

 flattened and no longer show the normal condition. 



When the spaces are examined they are seen not to be entirely 

 empty, but to contain here and there light pink-staining irregular 

 masses of a finely granular substance. This is apparently a pre- 

 cipitate derived from the licjuid existing within the spaces, and 

 was clearly seen at the period just before birth in all well-pre- 

 served fetal lungs not only of dog, but also of cat, rat and man. 

 The origin of the mucin constituent of the fluid is at least partly 

 from the goblet cells of the trachea, for in sections of the latter 

 we found them numerous and characteristically stained. Men- 

 tion may also be made of conspicuous large rounded cells lying 

 free within the spaces. They are distributed rather evenly but 

 not in large numbers, and usually occur singly. They measure 

 11 to 14 At in diameter, and have a single nucleus which is eccen- 

 trically placed. The nucleus may be round, oval or indented, 

 and varies from 6 to 7 /x in its greatest dimension. If flattened 

 in shape, the width is 3 to 4 ix. These cells in the dog have a 

 distinctly granular cytoplasm, which stains well with eosin. They 

 probably belong to the same variety of cells as those present in 

 the air-spaces of the normal breathing lung which take part in 

 removing carbon particles and other foreign matter from the 

 alveoli. In the fetal lung they may have the similar function of 



