The Evolution of Skeletal Knowledge: From Galen to Modern Understanding

The earliest comprehensive account of the skeletal system emerged around 180 AD in the writings of Galen, which served as the undisputed foundation for anatomical knowledge until the sixteenth century. Galen observed its protective and supportive functions, recognized the presence of hollow marrow within bones, and, based on its color, speculated that bones were formed from sperm. Andreas Vesalius, through his human dissections, notably in his seminal work "De Humani Corporis Fabrica" (1543), rectified many of Galen's erroneous descriptions. By the eighteenth century, a precise understanding of the human skeletal system had been established.

In the animal kingdom, there are two primary types of skeletal systems: exoskeletons and endoskeletons. Exoskeletons, often referred to as shells, serve to safeguard an organism's soft tissues from predators, offer structural support, and provide attachment points for locomotive muscles. They can also function as sensory organs, play roles in feeding and waste elimination, and, in terrestrial animals, act as a barrier against dehydration. The Cambrian period (542–488 million years ago) witnessed the emergence of many shell-forming organisms, coinciding with significant changes in ocean chemistry. The chemical composition of exoskeletons varies across species: insects, spiders, and crustaceans possess chitin-based exoskeletons, similar to cellulose; mollusks rely on calcium carbonate for shell hardness and strength; and microscopic diatoms use silica (silicon dioxide) to influence their buoyancy. When organisms with rigid exoskeletons outgrow them, a new exoskeleton forms beneath the old one, which is then shed through molting.

In an 1857 photograph, we see a human and a monkey skeleton alongside Reginald Southey (1835–1899), who was an English physician and the lifelong friend of the photographer, Charles Lutwidge Dodson, also known as Lewis Carroll (1832–1898).

Endoskeletons, on the other hand, are more robust than exoskeletons and accommodate the growth of larger animals. They provide essential support and protection, particularly evident in organisms like sponges and starfish, which would lack their characteristic shapes without these internal structures. Vertebrates possess endoskeletons composed of both bone and cartilage. In mammals, bones within the endoskeleton store calcium, and bone marrow serves as the production site for red and white blood cells. Chemically, bone is comprised of calcium hydroxyapatite, providing rigidity, and collagen, an elastic protein. An adult human typically has 206–208 bones, while the count varies in newborns, ranging from 270–350 bones.