the explanation has made the muscles that move the head to the sides smaller in quantity or fewer in number because the joint of the heads movement to the right and left is smooth and the reason for that is that it occurs from two extensions of the bones of the head that enter into two depressions of the first vertebra and not likewise the joint of its movement forward and backward because it occurs by the tip of the tooth entering into the depression of the first and not likewise the joint because if it were not more binding it would be exposed to dislocation more often and if the joint were smooth its movement would be easy and the muscles that are small in number and quantity would take the place of the large in number and quantity and not likewise what was from the joints to binding and the number of these muscles was made four because if they were only two their location would have to be between the front and back of the neck equally and if it were like that then if the head was moved to one side it would then remain unstable perhaps leaning forward and backward and its most inclination then would be to the front due to the increased weight of the front of the head so it was necessary to create two muscles on each side.
الشرح قد جعلت العضلات المحركة للرأس إلى الجانبين أصغر مقدارا أو أقل عددا وذلك لأن مفصل حركة الرأس يمينا وشمالا سلس وسبب ذلك أنه يحدث من زائدتين من عظام الرأس تدخل في نقرتين من الفقرة الأولى، ولا كذلك مفصل حركته قداما وخلفا، فإنه يحدث بدخول طرف السن في نقرة الأولى ولا كذلك مفصل فلو لم يكن أكثر إيثاقا لعرض له الخلع كثيرا وإذا كان المفصل سلسا كانت حركته سهلة فيقوم بها العضل القليل العدد والمقدار مقام الكثير العدد والمقدار ولا كذلك ما كان من المفاصل إلى الوثاقة وجعل عدد هذه العضلات أربعا لأنها لو كانت اثنتين فقط، لكان يجب أن يكون موضعها ما بين مقدم العنق ومؤخره على السواء، ولو كان كذلك لكان الرأس إذا حرك إلى أحد الجانبين يبقى حينئذ قلقا ربما مال إلى قدام وخلف وأكثر ميله حينئذ هو إلى قدام لزيادة ثقل مقدم الرأس فاحتيج أن يخلق من كل جانب عضلتان.