Anatomy 101: Directional Language and The Skeletal System

 Hey! Today we are going to talk about your bones, which make up your body's basic framework and constitute the Skeletal System. I have got another interesting thing planned for today's blog and that is learning about the basic anatomical directional terms. Let's start!

The Language of Doctors: Anatomical Language

Anatomical language is often referred to as the Language of Doctors, because it is how doctors talk to each other while describing a patient and also how they remember the names of a lot of body parts. Today, we are not going to talk about a lot of it, because trust me, there is a lot of it! We are going to step-by-step. Let's start with basic directional terms, shall we?

This is known as the Classic Anatomical Position where the body is erect and facing forward, with arms at sides and palms forward.

Now, imagine slicing that body into different sections or planes, you would get 4 planes.

When you slice it vertically, dividing it into left and right, you would get the Saggital plane. 

Now, imagine a plane parallel to that and you get the Parasaggital plane. 

The Coronal plane splits everything vertically into front and back.

The Transverse plane splits the body into 2 halves, slicing it horizontally.

Pay attention to this next one, as it will also help in understanding the Skeletal System. Anatomists divide the skeleton into 2 parts, The Axial and Appendicular Skeleton.

The Axial part of the body includes the Head and Trunk.

Whereas, the Appendicular part includes all the Limbs and the Pelvis, along with the shoulder blades.

Now getting to the actual stuff.

Anything which is in front of the body is known as Anterior or Ventral. Ex- Eyes, Nose, Stomach, Liver etc.

Anything which is in the back of the body is known as Posterior or Dorsal. Ex- Spine. Kidneys, Calcaneus which is the heel bone etc.

Combine those 2 terms and you get a way to describe the location of your heart. You would describe it as being anterior to, meaning in front of, the spine and posterior to, meaning behind the, sternum or breastbone. I know, lots of terms! The language of anatomy is vast, because it is basically another language for describing your body.

Anything which is at the top of your body is Superior, or Cranial.

Anything at the bottom of your body is Inferior, or Caudal.

Combine those 2 terms and you get a way to describe the location of your eyes. You would describe them as being superior to your nose but inferior to your brain, as they are below the brain and above the nose.

If you imagine a straight vertical line running towards the middle of your body, parts close to the Midline are called Medial. Parts away from the midline are called Lateral.

Now, looking at your appendages, or limbs. Parts closer to the trunk are called proximal. 

Parts farther away from the trunk are called distal. 

Combine those terms and you can describe the location of your thumb. You would simply describe it as being distal from the elbow. That is why its known as the Distal Phalanx. Phalanx is one bone out of the 14 phalanges in the fingers. Its distal because there are 2 bones in the thumb, 2 phalanges. One is proximal (closer to the palm) and one is distal (farther from the palm).

Pop Quiz! Imagine a toothpick gets stuck in someone's esophageal region and they need surgery to get it removed. How would a doctor describe a surgeon the anatomical location of the toothpick?

Answer: It would be described as being along the Medial line.

Posterior to the heart. 

Anterior to the vertebrae. 

Inferior to the collarbone. 

Superior to the stomach.

Common examples where doctors remember the positions of body parts by using these medical terminologies are as follows:

Superior and Inferior Mesenteric Arteries. Lateral Collateral Ligament and Medial Collateral Ligament (LCL and MCL). 

We are going to dive deeper into anatomical language in my next blogs.

The Skeletal System

As you might remember from my last blog, where we talked about tissues, turns out bone is nothing but a connective tissue. Also known as Osseous tissue, it has 2 types which we also discussed in the last blog:

1. Spongy Bone Tissue

2. Compact Bone Tissue

Instead of listing all the bones in the body — which, let’s be honest, can get a little dull (even though I do know them all, not to brag) — I’d much rather dive into the actual physiology of bones. So, lets get into it!

Shapes of Bones

Your bones come in all types of shapes and sizes to make them suitable for acheiving their task, which is providing you with a framework and connecting everything in the body, and also obviously helping you move. 

The human skeleton is divided into 2 parts- The Axial and Appendicular Skeleton, as I have mentioned above in this blog. The shapes of different types of bones in the body are as follows:

• Long Bones- These are your classic looking bones, the ones which come to mind when you think about bones. Ex- The bones of the limbs, such as the Humerus, Femur etc.

• Short Bones- These are kind of cuboidal shaped bones, found in the carpals of the hands and tarsals of the feet. Ex- Scaphoid, Cuneiform etc.

• Flat Bones- These bones as their name suggests are well, flat. Ex- Sternum (Breastbone), Scapula (Shoulder Blade).

• Irregular Bones- These bones are irregularly shaped and tend to be more specialised to give support to the structure they are found within. Ex- Pelvis, Vertebrae.

Now that you know all the shapes that bones come in, we can actually talk about their physiology and as you might know, in biology, form follows function,  just like how Anatomy is followed by Physiology. Just like the shape and size of every epithelial tissue serves a function, the shape of every bone is dependent upon its needs.

Composition of Bones

This is what a bone actually looks like under a microscope!                                                          

The more you zoom into the micro anatomy of bones, the more detailed and clear look you get into how they function at the cellular level. The structure of short, flat and irregular bones looks like a hard outer layer of Compact bone, with an inside covering of Spongy Bone. Bones are made of dense collagen fibers running around them and consist of Bone Marrow. Bone Marrow is of 2 types:

1. Red Bone Marrow- This type of bone marrow is where hematopoiesis or Red Blood Cell production takes place.

2. Yellow Bone Marrow- This type of bone marrow helps store energy as fat and also helps store calcium phosphate deposits.

The structure of long bones is different as it contains a concentration of spongy bone tissue and red bone marrow at its tips also known as Epiphyses. The epiphysis (singular for epiphyses) is also where you will find trabeculae, which is what spongy bone tissue is mostly made of. Trabeculae are also what were seen in the microscopic image of the bones I showed you. Compact bone tissue and Yellow bone marrow are found in the middle, where the trabecular network is present. This whole middle part of the bone is called Diaphysis.

Bone Structure

The basic structural units of bone are called Osteons (Haversian Canal). They are cylindrical and run parallel to the bone's axis. 

They are composed of tubes inside of tubes known as Lamellae. 

These tubes are filled with collagen fibres running in the same direction but if you look into the fibres of other lamellae, you will that they run in the opposite direction. Why is that you ask? That is because the bones need to be able to handle something known as Torsion  Stress, which is like twisting of the bone in weird ways.

The Central Canal inside each Osteon contain nerves and blood vessels, as this tissue needs nourishment too. 

Between the layers of lamellae, you can find the Lacunae. The lacunae are the actual work, because they house your Osteocytes. 

If you remember anything from my last blog, then you know that osteocytes are mature cells, which mantain the extracellular matrix of the tissues. 

And if you remember that, then you also probably remember that immature cells of the bone, called Osteoblasts, which form the extracellular matrix by secreting the ground substance and performing a special task. Again, mentioned in my old blog were Osteocasts which actually break down the extracellular matrix, in order to build it again. 

Now, with all that in mind, we can talk about Bone Remodelling.

Bone Formation 

Bone usually starts off cartilage in the embryonic phase (undifferentiated phase as you might remember from my old blog) and yes, it is still made from mesenchyme, that is why it is a Connective Tissue.

When the Osteoblasts come in, they secrete collagen along with enzymes that absorb calcium and phosphate as well as other from the blood. When the calcium phosphate crystallizes on the surface of the cartilage, bone is officially formed.

Bone Remodelling

In the last paragraph, we talked about how bone forms from an embryonic phase, now we talk about how bone forms again, when you get a fracture or some sort. 

When you do get a fracture, the Osteocytes initiate the process of bone remodelling by calling the osteoclasts and telling them to start breaking the bone! The osteoclasts secrete a collagen-digesting enzyme and an acidic hydrogen-ion mixture which sends the calcium phosphate stored in the marrow back to the blood, and break down collagen fibers. 

Once they're done, they undergo Apoptosis, which is programmed cell death, similar to necrosis, which is traumatic cell death. 

They do this so they don't break down the bone any further than they have to. This process is known as Resorption.

After that, the Osteoblasts come in and start building the bone again by secreting the ground substance and all that and this is a loop. In fact, your bones, do this so much, you actually get a new skeleton every 6-7 years! If you exercise more or injure yourself, those osteoclasts will break down the bone more and Osteoblasts will have to make it again. By doing this, you increase the ratio of Osteoblasts to Osteoclasts and get more bone strength.

There you have it! Now, you officially some knowledge about anatomical language, which let me tel you again is very very vast. So, we will have to take some extra blogs to cover that up, but we have officially covered basic directional terms. You also learnt a lot of stuff about bones in this blog which I intentionally left out in my old blog because then it would have been too long and I needed content to write about the Skeletal System.  

By,

Deeparsh Bhanot