None taken, I assure you! Let me offer a few direct quotes, here taken from Principles of Anatomy and Physiology by Tortora & Anagnostakos (a common textbook for Pre-Med/Nursing students):
*The first step toward increasing lung volume involves the contraction of the respiratory muscles-the diaphragm and external intercostal muscles. The diaphragm is THE SHEET OF SKELETAL MUSCLE THAT FORMS THE FLOOR OF THE THORACIC CAVITY(my emphasis, for clarity). As it contracts it moves downward, thereby increasing the depth of the thoracic cavity. At the same time, the external intercostal muscles contract, pulling the ribs upward and turining them slightly so the sternum is pushed forward. In this way the the circumference of the thoracic cavity also is increased.
The overall increase in the size of the thoracic cavity causes its pressure, called *intrathoracic* or *intrapleural pressure* to fall far below the pressure of the air inside the lungs. Consequently the walls of the lungs are sucked outward by the PARTIAL VACUUM. Expansion of the lungs is aided by the pleural membranes. The parietal pleura lining the chest cavity tends to stick to the visceral pleura around the lungs and to pull the visceral pleura with it.<%SNIP%>
Inspiration is frequently referred to as an *active* process because it is initiated by muscle contraction...<%SNIP%>
Breathing out, called expiration or exhalation, is also achieved by a pressure gradient. But this time the gradient is reversed so that the pressure in the lungs is greater than the pressure of the atmosphere. Expiration starts when the RESPIRATORY MUSCLES RELAX and the size of the chest cavity decreases in depth and circumference.<%SNIP%>
EXPIRATION DURING REST is basically a PASSIVE PROCESS, since no muscular contraction is required. However, the INTERNAL INTERCOSTALS DO AID IN EXPIRATION, ESPECIALLY DURING EXERCISE.<%SNIP%>
The pleural cavities are sealed off from the outside environment and cannot equalize their pressure with that of the environment. NOR CAN THE DIAPHRAGM and rib cage move inward enough to bring the intrathoracic pressure up to atmospheric pressure.*
<%SNIP%>The respiratory center has connections with the cerebral cortex, which means we can voluntarily alter our pattern of breathing. We can even refuse to breathe at all for a short period of time. Voluntary control is protective because it enables us to prevent water or irritation gases from entering the lungs. The ability to stop breathing is limited by the buildup of C02 in the blood, however. When the C02 increases to a certain level, the inspiratory center is stimulated, impulses are sent to inspiratory muscles, and breathing resumes whether or not the person wishes.It is impossible for people to kill themselves by holding their breath.<%SNIP%>end quote taken from the chapter on *The Respiratory System*.
And from the chapter on The Muscular System:
*Diaphragm dia=across, phragma=wall*
Origin is at the Xiphoid process, costal cartilages of the last six ribs, and lumbar verterbrae, Insertion is at the central tendon.
Forms the floor of thoracic cavity; contraction pulls central tendon downward and increases vertical length of thorax DURING INSPIRATION.*
And, once again, from the chapter on The Respiratory System:
*Coughing is a modified respiratory movement, preceded by a long-drawn and deep inspiration that is followed by a complete closure of the glottis, resulting strong expiration suddenly pushes glottis open and sends a blast of air through the upper respiratory passages. Stimulus for this REFLEX act could be a foreign body lodged in the larynx, trachea, or epiglottis.*end quote taken from the chapter on *The Respiratory System*.
Ergo, Rick, the diaphragm, though it IS a muscle, is NOT strong enough to do ANYthing by itself(I had this same argument with my A/P professor, except I took the side you're currently taking, and was cut to shreds by his dissection and demonstration of just how flimsy the diphragm IS!<G>). Also, it doesn't have a role in EXpiration(that's taken over by the intercostals--the muscles between the ribs, and can be enhanced with back and abdominal support, as demonstrated in the *wedge* type of breath support).
And, it isn't literally an *involuntary muscle* either, exactly. By definition it's one of the skeletal muscles, which are often viewed as being *voluntary* muscles, as opposed to *smooth* or *cardiac* muscles...
One final note, also from the chapter on The Respiratory System:
*Deep, or abdominal, breathing is called _diaphragmatic breathing_. It consists of the outward movement of the abdomen as a result of the contraction and descent of the diaphragm.*end quote taken from the chapter on *The Respiratory System*.
But, again, this is the INSPIRatory, or INward movement of the air that is affected by the diaphragm, and not the exhalation.
I wish you could have been there to see my face by the time my prof finished laying me out in lavender on this one<!>....It made dealing with conductors and contractors seem fun by comparison!<G>...