<prompt> 75 respiration is due to:DiaphragmInternal intercostalsIntercostalsSerratus anterior  <response>: Ans. A DiaphragmRef: Snell Anatomy, 7h ed. pg 499; BDC 6th ed. vol. I pg. 212-13 Diaphragm is major respiratory muscle Inspiration is an active process and requires contraction of inspiratory muscles. During quiet breathing expiration is a passive process. During inspiration all the 3 dimensions of the thoracic cavity expand Vertical-75 by diaphragm AP 25 external intercostals and internal intercostals Transverse Increase in vertical diameter is brought about by downward movement of diaphragm--which account for 70 of the thoracic expansion. 