RESPIRATION. 
203 
cle are attached by digitations to the cartilages of the eighth 
and all the posterior ribs, but the two last, to the tip of the 
sternum, and by its cruree to the inferior part of the bodies of all 
the vertebrae. When the fibres of this muscle are contracting, 
those of the intercostals contract consentaneously ; and although 
co-operating in expanding and enlarging the chest, they act at 
the same time as antagonists to the diaphragm, by offering fixed 
points for that muscle to act upon, and become a flat plane, in- 
creasing the capacity of the chest in a very considerable degree. 
This is readily observed by the pressure of its posterior surface 
upon the viscera of the abdomen, rendering the abdominal walls 
more prominent. The intercostal muscles, situated as they are 
between the ribs, would, by their contraction, draw them towards 
each other, and diminish the capacity of the chest, acting as ex- 
piratory instead of inspiratory agents. But viewing the form of 
the ribs, their attachment to the dorsal vertebrse, to their carti- 
lages, and these by acute angles to the sternum, throughout all 
the false ribs, by a dense fibrous lacing of the cartilage of one 
rib to that of its antecedent, it will be clearly seen the contraction 
of the interstitial muscles must carry the ribs forward and out- 
ward, increasing the dimensions of the thorax in width and 
depth at every part. Quiet inspiration requires no aid to the 
diaphragm but that of these muscles, which do assist in the ex- 
pansion of the chest at all times : however, when inspiration is in- 
creased in number and extent, the other auxiliary muscles are 
brought into action. The large serratus muscles, by their attach- 
ment to the cartilages of the eight anterior true ribs, assist in 
expanding the chest in hurried respiration, the scapula being 
rendered the fixed point for the posterior portion of the muscle to 
act upon. The superficial costal muscles, by their attachments 
to the ligamentum subflavum, and to the prominent part of the 
ribs near their middles, must at once shew their prescribed office 
to be that of raising and forcing the ribs outward, and so dilate 
the chest. Lastly, we have the transverse costal muscles, attached 
as far forward as the transverse processes of the last cervical ver- 
tebrae, taking their course backward over the upper part of the 
ribs as far as the last one, giving attachments to all of them at 
their posterior edges, the vertebrae being fixed points, with two 
or three of the anterior ribs nearly so. By contraction of their 
muscular fibres, the angles of the ribs will be drawn forward and 
outward, and co-operate in dilating the chest. In tranquil ex- 
piration, muscular exertion is little demanded, the resiliency 
of the lungs and walls of the chest, and the balancing power of 
the atmosphere, are efficient for the purpose of restoring the 
chest and lungs to the state of momentary repose — the assistance 
