>>7628625 (OP)Start with the overall form, which is somewhat like an egg but flattened front-to-back and with the narrow end less pointed. Seen from the side, this flattened egg is tipped back slightly.
Cut a slice off the top of this egg to represent the the opening at the top of the rib cage where the first ribs sit at the base of the neck. This slice should angle upwards from front to back, as do the actual ribs. Put another way, the base of the neck is lower in front than in the back. Seen from the front, it appears as an ellipse, about half the width of the egg.
See how, in the OP pic, an implied line is formed where the cartilaginous part of each rib attaches to the bony part? This line runs down the length of the ribcage on either side of the sternum. It marks the separation of the front plane from the side plane of the rib cage, and is where the ribs begin turning upward in direction as they go around towards the spinal column in back. Draw these two lines on your egg, diverging slightly as they descend; the space between them is the front plane of your rib cage, and the space on each side belongs to the side plane. Notice the side plane faces forward somewhat, which is why you can see it from the front. If you look at the ribcage from above, you'll see it is wider in back than in front, somewhat like a trapezoid, so the sides face forward. Interestingly, the opposite is true of the pelvis (wider in front than in back).
Finally, mark a point a little more than half-way down the center of the egg. This marks the bottom of the sternum and, in front, is on a level with the 5th rib. From here, draw the thoracic arch as a pair of lines descending to meet the previously drawn lines (the ones separating the front and side planes) at the bottom of the egg. Where these lines meet at the bottom marks the tip of the 10th rib.
(cont'd)