Medullary cavity
Cavitas medullaris
Also known as: marrow cavity
The central cavity of a long-bone shaft, lined by endosteum and filled with bone marrow — red in infants, mainly yellow (fatty) in adults.
Overview
The medullary cavity is created during endochondral ossification as osteoclasts resorb the primary spongy bone in the centre of the diaphysis, and enlarges throughout growth as endosteal resorption keeps pace with periosteal apposition. It lightens the bone without significant loss of bending strength.
At birth all marrow is red (haematopoietic). From about age 5–7 the marrow of the limb-bone shafts is progressively replaced by yellow marrow (adipocytes), so that by adulthood red marrow persists mainly in the axial skeleton, girdles and the proximal ends of the femur and humerus. Yellow marrow can reconvert to red marrow in chronic haemolysis or hypoxia. The cavity is perfused by branches of the nutrient artery that supply the marrow sinusoids and the inner cortex.
- Location
Within the diaphysis of long bones.
Parts & subdivisions
- Adipose tissue that stores fat in adult long-bone shafts.
- Lining membrane of the cavity.
Function
- Houses bone marrow (haematopoiesis or fat storage)
- Reduces bone weight
- Contains the medullary (nutrient) vascular network
Clinical significance
Fat embolism syndrome
After long-bone (especially femoral) fractures, marrow fat enters the circulation: within 24–72 h hypoxaemia, confusion and a petechial rash.
Intramedullary nailing
Femoral and tibial shaft fractures are commonly fixed with a nail inserted along the medullary cavity.
Compiled from standard anatomy and physiology references — see the Skeletal System sources. For education only; not medical advice.