The Skeletal System: How Skeleton Works

How the skeleton works

The Skeleton System:
01. Cranium
02. Cheek bone
03. Sinus
04. Orbit
05. Facial bones
06. Lower jaw joint
07. Teeth
08. Mandible
09. Ear drum
10. Scapula
11. Shoulder joint
12. Humerus
13. Elbow joint
14. Phalanges
15. Metacarpal bones
16. Carpal bones
17. Radius
18. Ulna
19. Sternum
20. Costal cartilages
21. Penis
22. Patella
23. Phalanges
24. Metatarsus
25. Tarsal bones
26. Tibia
27. Fibula
28. Femur
29. Coccygeal vertebrae
30. Pelvic girdle
31. Lumbar vertebrae
32. Thoracic vertebrae
33. Cervical vertebrae
34. Axis
35. Atlas

The skeleton is a system of bony levers moved by muscles which are anchored at crucial points on the bones. The bones are linked together at joints which act like shock absorbers. Bones have a complicated structure which gives great stability and yet allows movement. They are anchored by ligaments which permit a given degree of movement in specific directions. Each joint is surrounded by a joint capsule which contains the joint lubricant, synovial fluid. The ends of the bone involved in the joint are covered in case of a smooth surface which helps the joint move easily and helps to absorb any concussion as the dog’s weight comes down on the leg.

How bones grow and develop

Long bones begin in the foetus as cartilage structures, which are replaced by true bone in the latter weeks of pregnancy. A limb bone can be considered as a tubular structure with a joint or articulation at each end. The parts of the bone shaft not involved in the joint are covered with a tough, fibrous periosteum. In young, growing dogs, the inner layer of the periosteum is actively growing and producing bone, increasing the diameter of the bone. On the inside of a tubular bone, to prevent it becoming too thick and heavy, the older bone is reabsorbed and remodeled, keeping the actual bony wall or cortex the same thickness. Once the dog has stopped growing the periosteum becomes relatively inactive, although if a fracture occurs and needs repairing, it can become active again in that area. To avoid this process weakening the bone, the inside is filled with fine bony struts or trabecullae. The spaces between these are filled in the young animal with bone marrow, replaced by fat, as the dog gets older.
Growth in length occurs in regions of the bone near the joints called growth plates or epiphyseal plates. These growth plates are areas where cartilage is still being produced as an advancing layer behind the growth plate, in the metaphysis. The cartilage is being converted to bone, and so the bone grows in length. In most dogs all growth in length of long bones is complete by ten months of age.

Fuel for growth

Bone growth requires fuel, and this is provided by blood vessels. The main shaft of each bone is supplied by one or two large nutrient arteries which enter the bone through a hole in the shaft, the nutrient foramen. The epiphysis recieves blood from a ring of arteries inside the joint capsule. These arteries penetrate the whole of the epiphysis to feed the growing bone. They also supply nutrition to the inside layer of the articular cartilage; the rest of its nutrition comes from the synovial fluid inside the joint.