RESPIRATORY SPACES OF THE LUNGS 387 
In the first part of the alveolar ducts in the lungs of the 
human, dog, cat, etc., the fiber rings are rich in the three kinds 
of fibers and they do not constantly adhere to the described 
relationship. In the alveolar walls of mammals the muscle 
fibers are usually absent, while the thicker elastic and reticular 
fibers accompany one another. The muscle fibers are even 
absent in the mouths of the alveolar ducts and alveoli in such 
animals as the mole and guinea-pig, while the bat has no muscu- 
lature at all in the respiratory spaces. In short, it is seen in 
mammals that the thicker elastic and reticular fibers always 
accompany one another, but muscle fibers are not necessarily 
present. 
MEMBRANA PROPRIA AND ALVEOLAR PORES 
The elastic and reticular fibers are the principal tissues in the 
structure of the alveolar walls of mammalian lungs. These 
fibers are enclosed by a thin membrane which is called membrana 
propria. This membrane is usually considered as structureless, 
but Schulze and others state that it is slightly striped. Sussdorf 
and Miiller believe that membrana propria (which they call 
alveolar membrane) is an elastic membrane. All of the proofs 
of this point given by Miiller hold more true for the elastic fibers 
in the transverse section of the alveolar wall; he may have mis- 
taken these fibers for an elastic membrane. I agree that the 
membrana propria is structureless. The striping seen by Schulze 
was probably caused by the presence of the fibers enclosed in the 
membrane. 
It has already been mentioned that there is almost no mem- 
brane in the intercapillary spaces in the lungs of the mole and. 
bat, but deducing from the fact that various fibers run along the 
capillaries, there must be a membrane to enclose these fibers, 
although it is impossible to inspect it directly because of its 
exceeding thinness. 
The subject of alveolar pores has been one of much discussion. 
Up to the present it has not been decided whether or not alveolar 
pores are present in the alveolar walls of normal mammalian 
lungs. The detailed literature on this problem may be found 
