the leg needs lightness for movement and strength because it carries what is above it. as for the thigh its need for strength is greater because it carries what is above it transporting the leg and what is below it and its movement is little while the movement of the leg is great. therefore the aspect of strength was taken into account more in the thigh so it was created very large. in the other two it was taken into account so it was created with medium bone. it should have been one bone to be lighter and stronger but the joint of the foot cannot be made of one bone unless it is very thick as we will explain later. this is something that the leg cannot bear so it was necessary for its lower end to be made of two bones and its upper end does not need that so it was created from one bone and it was necessary for one of its bones to be disconnected.
الشرح الساق يحتاج إلى خفة لأجل الحركة، وإلى قوة لأنه حامل لما فوقه، وأما الفخذ فحاجته إلى القوة أكثر لأنه حامل لما فوقه ناقل للساق، وما دونه وحركته قليلة وحركة الساق كبيرة، فلذلك روعي في الفخذ جانب القوة أكثر فخلق عظيما جدا، وروعي في الآخران يخلق متوسط العظم. وكان ينبغي أن يكون عظما واحدا ليكون أخف وأقوى ولكن مفصل القدم لا يتأتى أن يكون بعظم واحد إلا أن يكون ثخينا جدا كما نبينه بعد. وذلك مما لا يحتمله الساق فاحتيج أن يكون طرفه السافل من عظمين وأعلاه مستغن عن ذلك فخلق من عظم واحد، واحتيج أن يكون أحد عظميه منقطعا.