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Cavities of the body

Learn here the cavities of the human body.

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Have you ever thought about what our bodies might look like if we remove all our major organs from them? I mean, what if we chuck out our brain or our spinal cord or got rid of our lungs and heart or even dispose of our stomach, intestines and liver? Yeah, I know what you’re thinking. Okay, fair enough. To someone who’s alive, removing their organs is not really a wise idea, but what about someone who isn’t alive anymore? I’m sure you’ve heard of the term ‘autopsy’ before – a special medical examination of a dead or deceased body to determine the cause of death.

In most cases, an autopsy involves an internal examination of the body and this involves a large Y incision being made into the anterior wall of the torso, so all the major organs can be removed for individual examination. Similarly, if the brain needs to be examined, the top part of the head known as the skullcap is removed to allow it to be extracted. So, what are we left with when all of these organs are removed?

Well, we have some large empty spaces, that’s for sure. Rooms inside our body which house our internal organs keeping them safe and secure and it’s the anatomy of these rooms that we’ll be studying today. So, welcome to our exploration of the cavities of the body.

Before we learn about the anatomy of the major body cavities, I guess it’s pretty important for us to first ask the question, what is the function of these spaces?

We’ll, firstly, they protect many delicate organs from getting damaged when we move. Some cavities provide this protection by means of strong walls made of bone, and the best example of this is, of course, your brain, which is safely tucked away inside your skull. Within body cavities, there’s often some type of fluid, and this helps to absorb shocks or allow for a certain degree of movement between adjacent organs or with the cavity wall, protecting them from damage. But we’ll speak more about this in just a few moments.

Secondly, the body cavities help to compartmentalize some of the important organs. Imagine if all the organs from your neck to your pelvis were able to move freely. You might end up in knots every time you did a cartwheel. This compartmentalization by body cavities can also help to limit the spread of infection. So, with all that in mind, we will look today at the five major cavities of the body examining their location, borders, contents, and some important features of each.

The first thing to note that all our main body cavities is that all of our main body cavities can be grouped by their location into two primary cavities, which are the dorsal cavity found along the posterior aspect of the body, and the ventral cavity, which is located more anteriorly. And we’re going to begin this tutorial by looking at the dorsal cavity.

So, the dorsal cavity is home to our central nervous system, and can be divided into two parts which are continuous with one another – the cranial cavity which, of course, houses the brain, and the vertebral canal, which provides a home for the spinal cord. And these cavities are continuous with each other through the foramen magnum, which is a large hole at the base of the skull and through which the caudal or inferior end of the medulla oblongata passes.

So, let’s start at the top with the cranial cavity.

The cranium is the most superior bony structure in the body and as a whole is composed of the bones of the skull excluding the mandible. And the cranium has several cavities and sinuses, so it’s therefore largely hollow, and we’re going to be focusing on the largest of these cavities which is the cranial cavity.

The borders or the walls of the cranial cavity are formed by eight cranial bones that make up what is known as the neurocranium – the part of the skull that contains the brain. So, let’s look at the cranial cavity from some different aspects now to get a better appreciation of this space.

So, first, we’re going to be looking at the top of the skull and this is called the calvaria. And the calvaria is made up of the superior portions of the frontal bone, the paired parietal bones, and the occipital bone, and these bones make up the superior border or the roof of the cranial cavity.

Looking up from a superior view of the base of the neurocranium, we can see that there’s a lot going on. There’s lots of foramina which are holes in the cranium through which arteries, veins, and nerves pass in addition to spaces which are known as fossae. And the most notable of these foramina, of course, is probably this one here which is the foramen magnum, which we mentioned earlier.

The bones that make up the floor of the cranial cavity can be divided quite nicely into three fossae, and these are the anterior, middle, and posterior cranial fossae. The anterior cranial fossa just behind the forehead is comprised of three bony elements – the orbital plate of the frontal bone, the superior part of the ethmoid bone, and the lesser wings of the sphenoid bone.

The middle cranial fossa is similarly formed by three bones – they’re the body and greater wings of the sphenoid bone, the paired temporal bones, and the inferior most portions of the paired parietal bones. And, finally, towards the back of the head, the posterior cranial fossa is comprised of the temporal bone, the paired parietal bones, and the occipital bone.

So just to recap, the bones that make up the borders of the cranial cavity were the frontal bone, the ethmoid bone, the sphenoid bone, the paired parietal and temporal bones, and the occipital bone. So, what lives inside this bony space then?

Well, it’s pretty common knowledge that the brain lives there. The brain being composed of the cerebrum, the brainstem, and the cerebellum, of course, but surrounding the brain are a number of protective layers, so let’s take a peek at this illustration to get a good look.

So now we’re looking at a cranial section of the skullcap or calvaria where we can see the skin and subcutaneous fat of the scalp here superficial to the bone and it’s the parietal bones that we can see here. The longitudinal fissure is running through the plane of the image between the hemispheres of the brain.

Underneath the bony walls of the cranial cavity, there are three protective layers of tissue and these are the meninges. From superficial to deep, they are the dura mater, the arachnoid mater, and the pia mater.

The dura mater is a double layered strong membrane and it is adhered to the bone and contains the venous or dural sinuses between its two layers. The middle meningeal layer is called the arachnoid mater and it’s a thinner membrane that covers the brain and has fine web-like projections that connect it to the deeper pia mater below, which are known as the arachnoid trabeculae. And it’s between these two layers that cerebrospinal fluid circulates and this fluid cushions the movements of the brain within this bony cavity.

The pia mater is a very delicate, thin layer of connective tissue that is so closely attached to the surface of the brain that it can’t be removed by dissection. The cerebrum, the cerebellum, and almost all of the brainstem are contained within the cranial cavity. The caudal part of the medulla oblongata peeks out the foramen magnum into the vertebral cavity.

Okay, so that’s it for the anatomy of the cranial cavity. Let’s follow that brainstem into our next cavity – the vertebral canal.

So we’re continuing in the dorsal cavity here down into the thin posterior vertebral canal which runs from the base of the cranium to the sacrum, and it travels in the midline of the body and contains the spinal cord, the roots of the spinal nerves, the vertebral meninges, and the blood vessels.

So let’s first define exactly what is meant by the vertebral canal.

So the bony elements of the vertebral canal are, of course, provided by the vertebrae – those knobbly bones you feel in your back and they look something like one of these depending on where in the vertebral column you’re looking. Regardless of their differences, most vertebrae bear many of the same landmarks. So, that bump which you can feel on your back is this structure here – the spinous process.

Each vertebra has an obvious hole in the middle called a vertebral foramen, and these foramina when stacked on top of one another make the vertebral canal, which houses the spinal cord. And there are various types of ligaments and other connective tissues holding the vertebrae together and intervertebral discs between the individual bones which we’ll look in closer detail in just a moment.

So what makes up the actual borders of this vertebral canal then? So, with the spinal cord located here within the vertebral canal, we can see this bony structure anterior to it which is called the vertebral body. Its posterior aspect forms the anterior border of the vertebral canal. An intervertebral disc is sandwiched between the vertebral body, so between the vertebrae and the intervertebral discs would be considered the anterior border of the vertebral canal.

Attached to the posterior surfaces of the vertebral bodies is the posterior longitudinal ligament running the length of the vertebral column and supporting the intervertebral joints. Laterally, the borders of the vertebral canal are incomplete which allows spaces to allow the spinal nerves to depart from the spinal cord into the peripheral nervous system. Where a bony wall exist, it’s formed by the pedicles of each vertebra and these are found between the vertebral body and these processes just here which are known as the transverse processes of the vertebra. And, finally, the posterior border of the vertebral canal is this piece of bone which is known as the lamina of the vertebra.

Okay, so, I’m going to switch to a different illustration now which shows a transverse section of the vertebral column in situ so that we can get a better appreciation of the contents of the vertebral canal.

So, of course, located centrally within the vertebral canal, we have the spinal cord out here which runs along the length of the superior three-quarters of the vertebral canal. Running along the periphery of the vertebral canal is a small space which is known as the epidural space, and this contains extradural fat providing some protection and cushioning to the spinal cord which is important given that the vertebral column is a somewhat mobile structure that moves as our core bends and flexes.

As the name suggests, this extradural fat is outside of the dural mater and continuous with the internal layer of dura in the cranial cavity, this meningeal layer also protects the spinal cord, and inside of it again is the arachnoid mater.

The arachnoid and the dura mater join together at the spinal rootlets and create a tight junction between the spinal nerves and the outside world. Cerebrospinal fluid fills the subarachnoid space and which, again, provides further cushioning and protection to the spinal cord.

So, just as we witnessed in the cranial cavity, we almost can’t see the pia mater here as it is so thin and closely knit to the spinal cord, but despite its fine and delicate structure, the pia mater plays an extremely important role in protection of the brain and the spinal cord.

And that basically sums up the contents of the vertebral canal for the majority of its length. That being said, the spinal cord undergoes some changes at approximately the level of the L1 or L2 vertebra so let’s take a closer look and see some more.

So, the spinal cord ends at L1 or L2 and becomes the cauda equina, which is effectively a bundle of spinal nerves that exits the vertebral canal at their respective level of the spinal nerve, and these nerves are surrounded by the dura and arachnoid mater into the sacrum while the pia mater travels the whole way to the coccyx as the filum terminale, acting as an anchor for the spinal cord.

Okay, so that’s it for both the compartments of the dorsal cavity. Let’s move on to the larger ventral cavity.

So, the ventral cavity is comprised of quite a few more cavities than its dorsal counterpart, and within this region we have the thoracic cavity which contains the pleural sacs, the superior mediastinum, and the pericardial sac which is found in the inferior mediastinum. The abdominopelvic cavity can be subdivided into the abdominal and the pelvic cavities, and I know this seems like a lot but fear not, it’s not as complicated as it looks. So, let’s start with the thoracic cavity.

As its name suggests, the thoracic cavity is the space contained by the thoracic wall which is composed of the vertebral bodies, twelve paired ribs and costal cartilages in addition to the sternum found anteriorly. The borders of this space are built largely as this cage-like structure created by these bony elements.

The superior border of the thoracic cavity is formed posteriorly by the vertebral body of the T1 vertebra, laterally by the first ribs, and anteriorly by the manubrium of the sternum. And the structures form what is known as the superior thoracic aperture, which is continuous with the neck or the cervical region of the body.

The inferior border of the thoracic cavity is marked by this large muscle which is known as the respiratory diaphragm which almost completely closes off the thorax from the abdominal cavity below. And, finally, the posterior and lateral anterior borders of the thoracic cavity are formed by the musculoskeletal walls comprising vertebral bodies, ribs, intercostal muscles and the sternum to create a bird cage-shaped space.

All of these structures make for a pretty unique structure that allows for both movement and protection and our respiratory action demands that the thorax is dynamic, but also houses some of the most important organs in our body such as our heart and lungs and must protect them.

The combination of bone for protection and support, cartilaginous joints for mobility, and intercostal muscles for movement enables expansion, retraction and flexibility without compromising on security.

The contents of this space as we mentioned are some of our most important organs and injury to them often puts our lives at risk. The main examples of such structures are the trachea, the bronchi and lungs, the esophagus, the heart, and the great vessels.

Within the thoracic cavity, you may often see two additional cavities contained within, and these are the pleural cavities and the pericardial cavity, and it’s important that we define these spaces as they are often misinterpreted, so let’s just take a moment to have a closer look.

So, there are two pleural cavities in the thorax – one surrounding each lung. While technically categorized as cavities, they don’t fulfill the true sense of the word in a sense. They form what is known as a potential space found between two membranes that surrounds the lungs. So, let’s just look at this illustration of the thoracic cavity for a minute to understand its location.

So as you can see in this image, the left lung has been removed and highlighted instead is the outermost lining of the lung, which is a strong fibrous membrane and this is known as the parietal pleura. Firmly attached to the outer surface of the lung is an inner layer of pleura which is known as the visceral pleura, and this is a serous layer – meaning it produces a thin membrane of fluid which lubricates the lungs and facilitates their gross movement.

The space between the parietal and visceral pleura is the pleural cavity and as I mentioned before, this is more of a potential space, meaning that in normal conditions, there isn’t actually a space per se. However, in the case of a pneumothorax or a hemothorax, air or blood can fill the pleural cavity.

The pleural cavity has two primary functions, both of which are extremely important to the process of breathing. So, firstly as I mentioned a moment ago, the lubricating serous fluid produced by the visceral membrane ensures a frictionless movement between the pleurae during ventilation, and without this, breathing would be very difficult or even very painful.

The second function of the pleural cavity is to prevent the lungs from collapsing and this concerns the pressure within the pleural cavity which is known as the intrapleural pressure, and this intrapleural pressure is less than that found in the lungs which is known as the intrapulmonary pressure. And this is really important so I’m going to say this one more time. The intrapleural pressure is less than the intrapulmonary pressure. And the lower pressure of the pleural cavities surrounding the lungs creates a vacuum which keeps the lungs expanded preventing them from collapsing.

So with all of that in mind, we can see how this cavity is a little bit different to the others we’ve discussed, but we can still make some of the same classifications as before. So, the borders of the pleural cavity are the parietal pleura on the external surface and the visceral pleura on the internal surface. And the contents of this space is pleural fluid.

The pericardial cavity is similar to that of the pleural cavity in that it is also a potential space, and it’s located in the midline of the thoracic cavity in a region known as the inferior mediastinum, just below the inferior half of the sternum as the name suggests, and it’s related to the pericardium of the heart.

So, we can actually use this illustration again to understand the pericardium. So, over here we have the fibrous pericardium which provides the outermost covering of the heart and this membrane is continuous the whole way around the heart and encloses it and the root of the great vessels and it’s also firmly attached to the diaphragm and the sternum anchoring the heart in place. And it also protects the heart acting as a physical barrier from pathogens.

So deep to the fibrous pericardium is the serous pericardium which is composed of two layers – a parietal pericardium which is fused and inseparable from the fibrous pericardium and a visceral pericardium which is attached to the outer surface of the heart itself and it’s also sometimes referred to as the epicardium.

The pericardial cavity is a potential space found between the two layers of the serous pericardium and it’s pretty similar to what we saw in the pleural cavity as it contains lubricating serous fluid known as pericardial fluid. Therefore, the pericardial cavity reduces friction during heartbeats by lubricating the surfaces of the serous epicardium facilitating free movement of the heart, and it also prevents overdilation of the heart when the volume of blood within the heart increases.

So, to sum up, we can define the borders of the pericardial cavity as the parietal pericardium and the visceral pericardium, and the contents of this cavity are pericardial fluid. Accumulation of excess fluid or bleeding into the pericardial cavity perhaps due to trauma results in a condition known as cardiac tamponade and this causes increased pressure within the pericardial cavity restricting the ability of the heart’s chambers to fill during the cardiac cycle.

One last region of the thoracic cavity which I want to explore is the superior mediastinum. So, the mediastinum as a whole is the central compartment of the thoracic cavity with the lungs located on either side, and it’s divided into superior and inferior compartments and the inferior mediastinum is where the heart can be found.

We’re going to take a very brief look at the superior mediastinum in this tutorial, but if you’re interested in learning more about it, check at our website for in-depth videos and articles on its anatomy. So, let’s look at these borders of the superior mediastinum to get our head around the area that we’re talking about.

So, the superior border is the superior thoracic aperture – the opening of the neck to the thorax at the level of the first rib – and the inferior border is an imaginary line drawn horizontally from the level of the fourth thoracic vertebra. And laterally, it is enclosed by the parietal pleura that we’ve seen previously. The anterior border is the manubrium and the body of the sternum, and the posterior border is the first four vertebral bodies.

So what can we find in this space? Well, for a relatively small region, there’s quite a large number of important anatomical structures to be found in the superior mediastinum such as the esophagus and the trachea in addition to the arch of the aorta and its three branches. Also found in this region is the superior vena cava and both the brachiocephalic veins and the arch of the azygos vein which is on the right side of the cavity. The nerves we can see here are the phrenic nerve and the vagus nerve and finally the remnant of the thymus.

Okay, so I think that’s enough about the thoracic cavity. Let’s move on with our journey through the vertebral cavity to the abdominal cavity.

So, this cavity is located directly inferior to the diaphragm which you remember was the lower border of the thoracic cavity making the diaphragm the superior border of the abdominal cavity. So, the abdominal cavity is limited anteriorly and laterally by the muscles and soft tissues of the abdominal wall and posteriorly by the vertebral bodies and muscles of the back.

The inferior border of the abdominal cavity is considered to be what’s known as the pelvic inlet which you can see just here, and this means that the parts of the pelvis superior to the pelvic inlet is considered to be part of the abdominal cavity and for that reason, the region of the pelvis is also known as the false pelvis.

The abdominal cavity provides a home for the gastrointestinal tract and several accessory digestive organs as well as the kidneys and the suprarenal glands.

Removing the anterior abdominal wall, we can see the liver here, the stomach, large intestine and part of the small intestine all tucked away under the greater omentum, and if we removed some of these, we can also see the pancreas and the spleen and the mesentery of the small intestine.

Deep to the mesentery in the retroperitoneal space are the kidneys and adrenal glands and the associated blood and nerve supply and lymphatic drainage.

And that’s the abdominal cavity – short and sweet.

Last but not least of our body cavities to explore today is the pelvic cavity, and as we’ve done with our other cavities, let’s begin by defining the borders of this final cavity.

As we just saw the boundary between the abdominal and the pelvic is defined by the pelvic inlet seen here which is formed by the boundary created by the pubic symphysis, the arcuate lines of the ilia, and the ala of the sacrum. The lateral margins of the pelvic cavity are comprised of the body of the ischium and posteriorly by the promontory of the sacrum.

The inferior aspect of the pelvic cavity is the pelvic floor which is a group of muscles attached to the inferior parts of the bony pelvis and this support much of the pelvic contents. This is a superior view looking down from above into the pelvic cavity, and the pelvic floor is primarily made up of what’s known as the pelvic diaphragm which contains the levator ani but also the coccygeus muscle as well as other soft tissues.

Of course, the contents of the pelvis are not quite as straightforward as the other cavities as there are obvious differences between the sexes. So we’ll start today with the female pelvis and here we can see the distal part of the sigmoid colon, the rectum, the ureters which have been cut, and the urinary bladder. and as well as these organs, we have blood vessels that supply both the pelvis and the lower limb and we can also see the vagina, the uterus, and the proximal end of the fallopian tubes in this image.

Other reproductive organs that are in the female pelvis are the remainders of the fallopian tubes and the ovaries and the broad ligament covering these structures.

In the male pelvis, again, we can see some of the same structures, for instance, the rectum and the distal end of the sigmoid colon, however, the overall space is a little bit smaller here due to the acute angle of the pubic symphysis, while the bladder is slightly larger than that of the female. Unique structures to the male pelvis are the seminal vesicles and the prostate which are posterior and inferior to the urinary bladder.

So before we finish, let’s take a look at a clinical condition that is related specifically to one of the cavities which we learned about today.

So let’s jump back to the pericardial cavity to have a look at what happens when an infection invades this cavity. So, pericarditis is defined as an inflammation of the pericardium which most often incurs in conjunction with other pathology of the heart or the chest. Most often, the cause of pericarditis is due to infection, meaning that it is more commonly experienced as an acute or short term condition. That being said, it can also occur chronically sometimes as a result of cardiac surgery or radiotherapy or even tuberculosis.

Inflammation of the pericardium causes a swelling and subsequent irritation resulting from friction between the pericardial layers, and this can be quite painful and often presents itself like a myocardial infarction or heart attack. It’s usually experienced with sudden onset which is particularly worse when lying down or breathing in. and the classic indication of pericarditis is a friction rub heard with a stethoscope during examination. And other physical signs may include fever, excessive sweating and shortness of breath. Of course, further examination with ECG, blood analysis and medical imaging will provide a more thorough diagnosis.

Treatment of pericarditis, of course, depends on the causative factor of the pericarditis, but most often aspirin or ibuprofen will be prescribed in addition to antibiotics and steroids if necessary. In some cases, a procedure known as pericardiocentesis will be done to remove excess fluid from the pericardial cavity to treat or prevent cardiac tamponade.

And that concludes our exploration of the cavities of the body for today. So let me briefly summarize what we talked about in this tutorial before I leave you.

So, we started with the cranial cavity and looked at a group of eight bones that made up the borders of the neurocranium which were the ethmoid, the sphenoid, the temporal bones, the parietal bones, and the occipital bone. The contents of this cavity were the cerebrum, the cerebellum and the brainstem, the meninges, the blood vessels, and also the cranial nerves.

We followed the brainstem out of the cranial cavity into the vertebral canal which is a cavity made by the foramina of the vertebrae stacked on top of one another and held together by ligaments and other soft tissues, and the contents of this space were similar to that of the cranial cavity beginning first, of course, with the spinal cord, the three layers of meninges, the vertebral blood vessels, and the spinal nerves.

We took an overview of the thorax in the thoracic cavity and we looked at the unique structures of the thoracic wall and how it is important in protection and functionality of the organs of the thorax and then we looked specifically at two unusual cavities within this space – the pleural cavities, one of which surrounds each lung, and the pericardial cavity around the heart.

We then looked at the pleural cavity which is the fluid-filled space between the parietal and visceral pleura and we looked at how these aided the movement of the lungs in the thorax. Then we moved on to the pericardial cavity which was similar to the pleural cavity consisting this time of a parietal and visceral pericardium surrounding the fluid-filled space and aiding movement of the heart.

We then descended into the abdominal cavity and took a whistle-stop tour of many organs found there and the borders of this large cavity, and finally, we looked at the pelvis which was continuous with the abdominal cavity and enclosed by the bones and the muscular walls of the pelvis, and we discussed the differences between males and females anatomy here.

And that is the end of our tutorial. I hope you’ve enjoyed watching it as much as we’ve loved making it. So until next time, thanks for using Kenhub, and happy studying.

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