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  •                 When the diaphragm contracts, its dome moves downwards into the abdomen, thus enlarging the thoracic cavity. Simultaneously, the inspiratory muscles move the rib cage upward and outward, also increase the volume of the thoracic cavity. This enlarges the volume of air within the lungs, pressure falls below atmospheric and air is drawn into the expanding lungs. While inspiration is an active process involving muscles, normal expiration is a passive event. As the thorax and lungs spring back to their original sizes, pulmonary air becomes temporarily compressed so that its pressure exceeds atmospheric and air flows from the lungs to the outside.
    • An extension of the head in relation to the cervical column would entail the passive stretching of the facial soft tissue layer draping the face and neck.The effect of this would be slight backward and downward forces exerted by the soft tissue layer on the facial skeleton thereby restraining the forward and increasing the downward component of the maxillary and mandibular growth relative to the cranial base.

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