Health and Nutrition

The Dog’s Internal Anatomy: Part 1 – Skeletal

The dog’s skeletal, muscular, and nervous systems are actually pretty similar to our own. They are mammals, just like us, and they share some of the same characteristics. Of course, they are four- legged, they do not sweat, they have a tail, and differ in many other ways, but the systems are alike.

Mammal structures are made up of bone, muscle, nerves, tendons, and ligaments. These all work together to make up the structure of the body and are what contribute to the motor functions.

I will be describing the make-up and the function of these structures, starting with bone in this part one issue.


Let me start with the skeletal system. Now, there are three different hormones that work to regulate the Calcium level in a dog’s body fluids, which you do not need to know the names of. Another important hormone is responsible for bone develop while dogs are in utero, or developing in the uterus.

I want to make this learning experience as user friendly as possible, so I will not bore you with scientific names and other long terms, unless I feel it is necessary. Even then, I will keep it simple and describe everything.

Bone Structure

Calcium, as we all know, is what makes bone so hard, but it is also vital to other functions in a mammal, our dog’s being mammals. Phosphorus is another important mineral that makes up bone.

Bone is actually rather lightweight material. Bone is made up of protein fibers called collagen, which mainly extend along the lines of tensional force. Collagen is weight for weight just as strong as steel. Calcium salts and phosphorus deposit themselves within these fibers. This mineral build-up is what gives bone its strength and rigidity. It is the collagen fibers that give bone its great tensile strength, while the calcium salts give bone its compressional strength.

The rigid skeleton is what provides protection for your dog’s vital organs, allows the dog to have locomotion, and supports your dog’s weight against gravity. However, when stresses such as excess weight are applied, the bone is placed under more pressure. Other stresses can include jumping up and down from high places, excessive running, and other high impact activities. These are contributing factors to bone and joint diseases found in dogs.

Bone has two types in dogs and other mammals. Cortical bone, or compact bone, makes up the outer layer of most bones, and makes up about 80% of the bone in the body. The other type of bone is called trabecular bone, or otherwise known as spongy bone. This type of bone makes up the other 20% of bone in the body. The spongy bone is found inside the compact bone.

Osteocytes are the bone cells that are found in compact bone. They have a low surface to volume ratio, and receive nutrients from tiny canals within the compact bone. These canals are called Haversian canals, which contain blood vessels which transport blood and nutrients to compact bone.

The spongy bone is made up of spicules, or plates. It is highly metabolic, has a high surface to volume ratio, with many bone cells sitting on the plate surface. Spongy bone gets its nutrients by diffusion through from the ECF. That is, the nutrients pass from the body’s extracellular fluid into the bone.

So, you can see that bone is a living tissue. Blood and nutrients flow in and out of bone, regulated by the body to maintain a constant level of minerals, blood, nutrients, and so on.

Bone Growth

I will talk about growth of long bones, which grow differently than the skull. I am focusing on a dog’s legs because this is what we are concerned with here. I want to teach you about your dog’s vulnerable structures when it comes to leg and joint problems common in dogs.

Long bones grow by first forming a cartilage base. Remember, that bone is made up of a protein called collagen. Ossification then occurs, that is, minerals bind to the cartilage, forming actual bone. The ossification process is what forms skeletal bone as we know it.

The ends of long bones are where growth occurs. Epiphyses or epiphyseal plates are formed at the ends of the shaft of long bones. These plates separate the ends from the rest of the shaft of the bone.

Growth occurs as long as the plates are present. Once the plates are closed, growth stops and the newly formed bone length is now part of the shaft of the bone. Hormones regulate the growth in width of the bone.

While bone growth is taking place, these areas of soft bone are vulnerable to injury, especially in the carpal (wrist) and stifle (knee).

Bone Formation and Resorption

Just because bones stop growing does not mean they become inactive. Far from it. Remember how blood and nutrients are transported to both types of bone? These systems are always present in bone.

Old bone does get constantly replaced and new bone formed, but the length of the bone will not change. This process is called resorption and formation. Resorption refers to the process of removal of old bone cells and re-assimilating them into the body. Formation, of course, refers to the process of forming new bone cells.

Bone marrow forms the cells that are responsible for the resorption and formation process. The cells that are responsible for eroding and resorbing old bone cells are called osteoclasts. The cells that are responsible for forming new bone cells are called osteoblasts.

Osteoblasts secrete collagen and other proteins, and various other substances to form bone. These cells eventually turn into Osteocytes. Remember these? Osteocytes are the cells that make up compact bone. Do you see how this works now?


I have tried to explain the skeletal structure of a dog in a way that you will understand it and have more knowledge in this area. I hope that I have succeeded. There is so much that occurs in your dog’s system that this is just barely the tip of the iceberg.

This information is crucial to understanding why it is so important to never let your dog, young or old, jump to and from high places, or otherwise place great stresses on their bodies.

While the proteins and minerals that make up bone are unbelievably strong, damage can occur.

Injuries can slow or impede the natural processes in their body to repair themselves. Some injuries may go unnoticed and not come up until later years. By then, it is too late. Please regulate your dog’s play and activities. For more information on keeping your dog healthy and feeling youthful, please visit

Author: Richard Delgado
Copyright © 2003-2005
CESS Enterprises
Pet Care 4 Our Animals

About the Author: Richard Delgado brings his 15 years of pet care experience to the internet community to teach all dog lovers to properly care for their dogs.

Note: This section of the Canada’s Guide to Dogs website is intended as a source of information only. It is not intended as a substitute for professional care. Always consult with your Veterinarian about health related matters.

— is an Amazon Associate as well as a participant in various affiliate programs, as such fees are earned from qualifying purchases.