Final answer:
All the muscles mentioned - external intercostals, scalenes, and the diaphragm - contribute to increasing the thoracic cavity volume during inspiration. Their contraction results in a decrease in intrapleural pressure and facilitates the ingress of air into the lungs.
Step-by-step explanation:
Contraction of which of these muscles increases thoracic cavity volume? The correct answer is D. all of these: A. external intercostals, B. scalenes, and C. diaphragm.
The diaphragm, when it contracts, it flattens and the volume inside the pleural cavities increases, leading to a decrease in pressure which results in air flowing into the lungs. The external intercostals are muscles that span between the ribs and help in changing the shape of the rib cage and the volume-pressure ratio inside the thoracic cavity during inhalation. Also, the scalenes, which are muscles in the neck, assist in elevating the first two ribs, thereby contributing to the increase of thoracic cavity volume during deep breaths.
In summary, contraction of all these muscles contributes to increasing thoracic cavity volume, which decreases pressure inside according to Boyle's Law, hence allowing air to rush into the lungs.