RESPIRATORY SPACES OF THE LUNGS 369 
ment in the reports of investigation has been the tendency to 
generalize the results which were obtained in a few animals. 
The materials were cut in sections 50 » to 200 u. In the fol- 
lowing animals, the mole, bat, rat, guinea-pig, which have smaller 
alveoli, thinner sections were used than in the cat, dog, and man, 
in which alveoli are larger. For staining I used besides van 
Gieson, hematoxylin-eosin, and Heidenhain’s iron-hematoxylin. 
The following description of the alveolar system is divided 
into three parts, viz., mouths of the alveolar ducts, mouths of 
the alveoli, and the alveolar walls. 
(I mean by the term ‘mouth of the alveolar ducts’ not only 
the mouth of each system of alveolar ducts, but also the place 
of division of each branch of the duct into smaller branches.) 
1. Muscle fibers in the mouths of the alveolar ducts. Stieda (in 
mammals) is of the opinion that the muscle fibers are missing in 
alveolar ducts. Schulze (Gn mammals) is of the contrary opin- 
ion, that the muscle fibers run circularly around the mouth of 
the first parts of the alvolar ducts. v. Ebner (in man) main- 
tains that the muscle tissue is to be found in the mouths through- 
out the ducts. According to Prenant (in mammals), the muscle 
fibers disappear in the periphery of the alveolar ducts, while, 
according to Stohr, they disappear in air sacs. 
According to my findings, the presence or absence of the muscle 
tissues varies with the kind of animals. The muscle fibers are 
entirely absent in all parts of the alveolar ducts in the bat lung. 
But there are existent some circular arranged muscle fibers at 
the mouths of the alveolar ducts in the mole, rat, guinea-pig, 
rabbit, goat, cat, dog, and man (figs. 25 and 26). The muscle rings 
consist of bundles of fibers at the mouths of the main alveolar 
ducts and of less or sometimes of single fibers near the periphery. 
In the mole and guinea-pig, the muscle rings are sometimes ab- 
sent at the mouths of the periphery of the ducts. As stated by 
Miller and confirmed by my study, the muscle rings in the mouths 
of the alveolar ducts of the cat lungs consist of bundles, fibers of 
which are well developed even in the mouth of periphery of the 
alveolar ducts. No remarkable difference can be seen in other 
animals. 
