372 ANIMAL PHYSIOLOGY. 
of these parts and that of the attached muscles, etc., assists in 
the return of the chest to its original position, entirely indepen- 
Fia. 303.—Dorsal view of four vertebree 
and three attached ribs, showing 
attachment of elevator muscles of 
ribs and intercostals (after Allen 
Thomson). 1, long and short eleva- 
tors; 2, external intercostal ; 3, in- 
ternal intercostal. 
dently of the action of muscles. 
Moreover, with the descent of the 
diaphragm the abdominal viscera 
have been thrust down and com- 
pressed together with their included 
gases; when this muscle relaxes, 
they naturally exert an upward 
pressure. Putting these events 
together, it is not difficult to un- 
derstand why the air should be 
squeezed out of the lungs, the elas- 
ticity of which latter is, as we have 
shown, an importaut factor in itself. 
The Muscles of Respiration.—The 
diaphragm may be considered the 
most important single respiratory 
muscle, and can of itself maintain 
respiration. The scaleni are important as fixators of the ribs; 
the levatores costarum, and external intercostals, as normal ele- 
vators. The quadratus lumborum 
assists the diaphragm by fixing the 
last rib. These, with the serratus 
porticus superior, may be regarded 
as the principal muscles called into 
action in an ordinary inspiration. 
The muscles used in an ordinary ex- 
piratory act are the internal intercos- 
tals, the triangularis sterni, and ser- 
ralus posticus inferior. In forced 
inspiration the lower ribs are drawn 
down and retracted, giving support 
in their fixed position to the dia- 
phragm. The scaleni, pectorales, 
serratus magnus, latissimus dorsi, 
and others are called into action; but 
when dyspncea becomes extreme, as 
in one with a fit of asthma, nearly all 
the muscles of the body may be called 
into play, even the muscles of the 
Fic. 304.—Laryngoscopic views of 
the glottis, etc. (after Quain and 
Czermak). I. Larynx in quiet 
breathing. II. During a deep in- 
spiration. In this case the rings 
of the trachea and commence- 
mentof bronchiare visible. Such 
a condition is persistent in many 
forms of disease in which respir- 
ation is attended with difficulty. 
face, which are not normally active at all or but very slightly 
in natural breathing. 
