the fourth benefit if it were created as a single bone there would be no opening in it through which would be interspersed the chest muscles that are essential for the respiratory organs and what is connected to them such as the voice. the explanation for that is that breathing has been shown previously as being compelled to do so and it is only accomplished by the movement of the lungs and the diaphragm expanding and contracting so that air is drawn in during expansion due to the impossibility of a vacuum and the excess of the spirit and what is heated from that air is expelled. so its benefit is nullified by contraction and the movement of contraction and expansion.
المنفعة الرابعة إنها لو خلقت عظما واحدا لم يكن فيه فرج يتخللها عضل الصدر المعينة في آلات التنفس وما يتصل به كالصوت وبيان ذلك أن التنفس قد دللنا فيما سلف على وجه الاضطرار إليه وهو إنما يتم بحركة الرئة، والحجاب انبساطا وانقباضا لينجذب الهواء عند الانبساط لاستحالة الخلاء، ويندفع فضول الروح، وما يسخن من ذلك الهواء. فبطلت فائدته عند الانقباض وحركة الانقباض والانبساط.