AIR SPACES IN LUNG OF CAT 469 
alveolaris, the air spaces connected with it, their blood vessels, 
lymph vessels and nerves. Collectively this forms the anatomi- 
cal unit of the lung. 
The acinus of Laguesse and d’Hardiviller is not the unit of 
structure; their unit is the lobule as defined by them and consists 
of from fifty to one hundred acini. 
Rindfleisch has given a description of the finer structure of the 
lung, which corresponds quite closely to that of Laguesse and 
d’Hardiviller, in which he says the acinus is the unit of the lung; 
‘that it is far more constant than the lobule. The lobule, com- 
posed of from twenty to thirty acini, is however, pathologically 
of much more importance. In other words he seems to make a 
distinction between an anatomical and a pathological unit. 
MATERIAL 
The present investigations are based on a study of the lung of 
the cat. In the preparation of the model I received material 
assistance from one of my students. Mr. G. H. Scheer. 
The block from which the reconstruction was made was cut, 
perpendicular to the pleura, into sections 20 u in thickness. The 
reconstruction ran through 115 sections of the series. The high- 
est point of the reconstruction was situated three and a half mil- 
limeters below the pleura; it can be said, therefore, that the re- 
construction represents the structure of the lung uninfluenced by 
the pleura. 
The dimensions of the reconstruction are 23 x 24 x 20 cm.; 
as the amplification was 100, the portion of the lung entering into 
the reconstruction was approximately 2.3 x 2.4x2mm. The 
amount of shrinkage was not estimated; as great care was exer- 
cised in the imbedding it probaby is a negligible quantity. 
In studying the air spaces of the lung, the positive model is 
by far the most useful. Corrosions and corrosion models (nega- 
tive models) of the lung do not exhibit its structure as clearly as 
do positive models. I have repeatedly called attention to this 
point and shown that the use of corrosions has led several inves- 
tigators into error. Early in my work I abandoned their use 
except for demonstrating the gross arrangement of the bronchi. 
