İKİNCİ FEN · İKİNCİ BÂB · ÜÇÜNCÜ FASIL · İkinci Madde
Second Article
Concerning the bones of the forearm.
O, esteemed one! Know that the anatomists have said: The forearm is composed of two long bones. They are called *zendeyn*. The one nearer to the thumb, situated above, is thinner and is named *zend-i a’lā* (the superior bone). The lower bone, nearer to the little finger, has been created thicker, being in a supporting position, and it is named *zend-i esfel* (the inferior bone).
The benefit of the *zend-i a’lā* is to turn the forearm downwards and to turn the palm upwards. The benefit of the *zend-i esfel*, however, is to loosen and tighten the wrist. The middle of each of these two bones has been created thin and soft, so that thick muscles may easily carry them, freeing themselves from their weighty bulkiness. But because their circumferences are not composed of fleshy muscles and are covered with ligaments, their ends have been made thick and durable to protect against friction arising from the movement of the joints. The *zend-i a’lā* is curved, its curvature beginning on the inner side and curving upwards in a fold. Its benefit is known to render the *zend-i a’lā* suitable for curved movements. As for loosening and tightening movements, the most appropriate position is straight, so the *zend-i esfel* has been created straight.
The elbow joint arises from the articulations of the humerus with the *zend-i a’lā* and *zend-i esfel*. There is a small depression on the side of the *zend-i a’lā*, to which the head, located on the upper side of the humerus, is attached. Rotation occurs through the turning of this head within that depression. The *zend-i esfel* has two extensions, between which there is a notch resembling the letter (sin). And the surface in its depression has been created convex. Thus, the humerus engages with the notch on the concave side, and the elbow joint joins with it. So when the inner notch moves over the depression’s notch backwards and downwards, the hand opens. When the surface of the notch separates from the depression holding the aforementioned head, it prevents further opening of the hand, and the bones of the humerus and forearm stand close to each other in direction. When the two notches move over one another forwards and upwards, the hand closes, and the bone of the forearm adjoins the humerus from the front and below. [119/a] The lower sides of the two extensions come together as if they were a single thing, producing a wide and common depression which is often seen in the lower extension. The portion remaining from this depression has been created round, flat, and soft like the palm of the hand, to be free from dangers. There is a vertical extension behind the depression of the *zend-i esfel*, its benefit being protection and preservation.
“We praise Allah who created these with wisdom and artistry. His glory is exalted.”