Biomedical Ephemera, or: A Frog for Your Boils

Bones of the human hand and wrist

At birth, the human infant typically has 270 bones, and many bones fuse together to reach a total of 207 bones in adulthood. However, some bones aren’t even formed yet in babies, when they’re born.

Included among those bones is the eight carpals, or wrist-bones. They begin to ossify (solidify) around 2 months of age, and continue to form through the beginning of puberty.

The last wrist bone to form (the pisiform) is a sesamoid bone, which forms part of the the ulnar border (near the pinky finger as opposed to the thumb) of the wrist.

Being a sesamoid bone means that it is a bone embedded within a tendon or muscle, and most often formed in response to strain. Unlike the other wrist bones, it doesn’t contribute to the rotation of the wrist, and is not an anchor for any major tendons or muscles.

Atlas and Text-Book of Human Anatomy. Dr. Johannes Sobotta, 1914.

Bone types

Top: Metacarpals (long bones) and carpals (short bones) Second row, left: Left ulna (long bone) Second row, right: Scapula and sternum (flat bones) Third row, left: Sagittal section of the knee joint, including the patella (sesamoid bone) Third row, right: Thoracic vertebrae (irregular bones) Bottom: Complete Skeleton

Bones are classified into five groups, organized by shape.

Long bones are longer than they are wide, and are subjected to most of the load-bearing responsibilities in everyday life. These include the humerus, radius, and ulna (arms); fibula, femur, and tibia (legs), as well as the phalanges (fingers and toes), metacarpals (hands) and metatarsals (feet).

They grow from the epiphysis (growth plate) at either end of the bone, and failure of these bones to grow causes the majority of dwarfism cases.

Short bones are as wide as they are long, and provide support, but do not bear heavy loads or move much. These include the tarsals (feet) and carpals (hands/wrists).

Flat bones are broad bones that provide protection to organs, and large areas for muscle attachment. These include the bones in the skull, the ilium, scapula, sternum, and ribs. The flat bones consist of two layers of compact bone, surrounding a layer of cancellous bone, where the majority of red bone marrow exists. In adults, most red blood cells are produced in the flat bones.

Sesamoid bones are bones within tendons, which pass over a joint. The most familiar sesamoid bone is the patella, or knee-bone. These bones provide protection to delicate joints.

Irregular bones don't fit into any of the above categories. The mandible and vertebrae are irregular bones.

Images:

Atlas and Text-book of Human Anatomy. Dr. Johannes Sobotta, 1914. Anatomy: Descriptive and Applied. Henry Gray, 1918. A Series of Engravings, representing the Bones of the Human Skeleton. William Cheselden, 1819.