elasticity of the lungs. gr 
It is composed of a succession of cartilaginous rings, which 
nearly form complete circles. Each ring is connected to the 
next to it in succession by a strong elastic membrane. A 
small segment is cut off from each of those rings behind, 
where the place of cartilage is supplied by the same strong 
elastic membrane by which the rings are connected. By 
this contrivance the rings may be expanded, and the diameter 
of the windpipe is rendered susceptible of variation as well as 
its axis. 
As soon as the windpipe has fairly entered into the con- 
fines of the chest, it divides into two branches, one of which 
proceeds to the right and the other to the left side. These 
branches possess the same structure with the windpipe itself, 
and each as it proceeds is divided into smaller branches, which 
again and again ramify into others still smaller. The branches 
for some time are formed like the trunk from which they 
proceed, but at length they drop the cartilaginous structure, 
and become simply membranous tubes. These tubes ulti- 
mately terminate in cells, between which there is a free 
passage for the air. The interstices between these tubes and 
cells are filled up by glands, the common cellular substance, 
and by blood vessels and nerves. The whole of this mass is 
enveloped in a fine membranous covering, termed the pleura 
pulmonalis, which however is impervious to the air. To 
each of the two original divisions of the windpipe a distinct 
lung is appended, called the right and left, between which 
there afterwards exists no communication. 
The lungs occupy that part of the animal frame denomi- 
nated the chest. This, in man, is the upper part of the trunk 
of the body. The shell of the chest is composed of bone, 
