The contraction of the diaphragm
Contraction of diaphragm makes it flat and increases the volume of thoracic chamber in the vertical axis during inspiration. Increase in the volume of the thoracic chamber decreases intrapulmonary pressure leading to inspiration.
The contraction of external inter-costal muscles lifts up the ribs and the sternum causing an increase in the volume of the thoracic chamber in the dorso-ventral axis.