Why are there two protruding bones on our backs?

Also known as scapula and pipa. Located at the back of the chest, it is a triangular flat bone between the second and seventh ribs. Divided into two sides, three corners and three sides. The subscapular fossa is a large and shallow fossa in front. There are transverse bone ridges on the back, called scapular ridges, supraspinatus fossa and infraspinatus fossa, called supraspinatus fossa and infraspinatus fossa respectively. The lateral side of the scapula is flat, which is called acromion. The lateral angle is thick and has pear-shaped articular surface, which is called glenoid. The upper corner and the lower corner are located at the upper and lower ends of the inner edge, respectively, and horizontally face the second rib and the seventh rib. It can be used as a sign of counting tendons. The medial margin is slender and faces the spine. The side edge is thick and faces the armpit. The upper edge is the shortest, and there is a finger-like protrusion near the outer corner, which is called coracoid process. A left and a right, slightly triangular.

The shoulder joint consists of the scapula, clavicle and humerus.