Sunday, 3 August 2014

Four-Armed Bipeds

     As everybody knows, the Hindu gods have multiple arms, and Edgar Rice Burroughs' Green Martians had four. But artists, all too often, are at a loss how to draw them. Indian painters are woeful at the task. The arms of their deities appear to fan out from somewhere on the back without any obvious joint or muscle attachments, and with the thorax the same length as that of a normal man. (Then again, perhaps gods are really built that way; they don't have to conform to human anatomic restrictions.) As for Burroughs' illustrators, well, artists are normally trained in basic anatomy so that they know how to depict the human body, but when it comes to imaginary humanoids, their skills fail them. Wave your eyes over this collection of illustrations. As you can see, some of them are reasonable, particularly those from the film, John Carter, but many of them depict the shoulders so close together they would get in one another's way. Others are really bad; they show a biped with two chests, one on top of the other, with what looks like space for a second set of lungs, and possibly a stomach.
     So, in the interest of future illustrators, not to mention Hindus, I shall explain how such a being would be put together.
     Here is a diagram of the human shoulder and rib cage. As you can see, I took it from a high quality book with thin pages, hence the text visible from the other side.
     Your attention is drawn to the clavicle, or collar bone, which connects the scapula, or shoulder blade to the sternum, or breast bone. This system is derived from the pectoral girdle of the first four-legged animals which crawled out of the primeval sea, with their limbs sprawled to the side as you see in lizards today. In order to provide a firm support for the body, and to keep the forelegs in place, the pectoral girdle formed a closed circle of bones connecting the breast bone to the shoulder joint and the shoulder joint to the backbone.
     Mammals have managed to evolve limbs which stand vertically rather than sprawl horizontally, and have lost the connection between the shoulder joint and the backbone. It is held in place by a sling of muscles. This need not be the case with a four-armed alien. Mammals such as horses, whose forelegs need to move only forward and backwards, have dispensed with the clavicle, but it is still required for climbing animals, to provide an anchor while they spread their arms horizontally.
     A four-armed biped would very likely possess a longer rib cage. In any case, it is virtually certain that the breast bone would extend the whole length of the rib cage. (As you can see, the lower five pairs of human ribs do not connect to it.) It is possible that the rib cage would not flair as much towards the bottom as that of a human. What you would definitely have is a second shoulder blade at the bottom of the rib cage, and a second clavicle attached to the other end of the breast bone, making the whole structure very tight. That V-shape empty of ribs which appears in many of the illustrations is definitely wrong; its place should be taken by a visible lower collar bone, and a second set of pectoral muscles. Simple.
     What about those multi-legged quadrupeds, described by Burroughs? The front two sets, next to the rib cage, would have shoulder blades as described, but not collar bones. The rear ones, not in proximity to the rib cage, would possess hip joints connected firmly to the backbone as a pelvic girdle, just as they are on earth. For extra strength, the front pelvic girdle might actually be connected to the rear one. The sequence in which these limbs moved would no doubt vary with the species, and particularly its length.
     Burroughs said that the Green Martians' second pair of arms could also be used as legs. In fact, I suspect that a six-limbed intelligent life form would more closely resemble a centaur than a human being.

     But, to a certain extent, these are evolutionary anomalies, which I shall deal with in my next article - unless you want to return to the Index.