Head and Neck
But not teeth, saliva or even salivary glands they get their own page
Ingestion occurs at the head, and Digestion begins in the head
Here we'll take a tour of the some of the things found in the head
Skull
The skull attaches to the spine at a hole called the Foramen Magnum. This allows the spinal cord to run directly through the pipe formed by the spine
The Foramen Magnum is slightly behind the center of gravity for the skull, rather than directly below it. This is part of our evolutionary heritage, as the Foramen Magnum has moved forwards, closer to the jaw, as we've changed from walking on all fours, to occasionally on fours, to fully bipedal. Because of the foramen magnum being behind the center of gravity, most classroom skeleton models, the skull tends to tip forwards. On humans, this is not a problem, as the major muscles of the neck attach behind the magnum foramen, pulling the skull upwards.
The skull has 2 main parts, the cranium and the face. The cranium is the space where the brain sits. The cranium completely encapsulates the brain, except for the Foramen Magnum, and smaller holes for the ears, optic nerve and large arteries.
The cranium consists of 8 cranial bones. these have sutures (firm joints) between them. As an adult these suture joints allow a very small amount of movement in a head knock, thus dissipating some of the energy and slightly reducing the risk of a skull fracture. While a person grows, these sutures also allow movement as the bones grow - I think of several flowers blooming next to each other - if they fuse while a persons skull is still growing, it can result in deformities - much like gluing the flowers together whilst they are still buds. These major sutures of the cranium are include the same locations where the baby has its fontanelles.
Fontanelles are large soft areas. These are full of elastic connective tissue. They connect the bones of the skull in the new born. Their purpose is to allow the skull to change shape during birth, so that it fits through the birth canal. After birth the skull bones continue to grow and the fontanelles shrink, until they disappear and their only evidence is the sutures. These soft areas are not entirely gone until the child is 18 months old. It is extremely important to be careful with a babies head, as it is far softer than an adults and the skull bones have not yet connected, thus their cranium is far more fragile.
The face consists of 14 bones. Of particular note is the Maxilla. This is the top part of the jaw which includes the nasal cavity.
Also, the Mandible - this is the movable part of the jaw, used for chewing and talking.
Jaw = Mandible
So, the top of the mouth is the Maxilla, the jaw is the Mandible and to chew is to masticate
Chewing = Mastication
The mandible has large muscles attaching it to the skull. The biggest of these muscles is the Temporal muscle. You can feel this muscle both on the jaw and on the skull - at the temple and above, when you bite down had. It attaches to the top of the jaw and the other end attaches to the skull. It runs behind the portion of your cheek-bone that goes from your cheek to your ears. In a gorilla, and primates that eat a lot of vegetation, this muscle goes all the way to the top of the head. Furthermore, it is so strong in gorillas that the top of their head has a bony bit called a sagittal crest, to give the muscle even more area to attach to. To see why this would be important for a raw plant based diet, eat 5 raw and uncut carrots - it'll be quite the workout (note, the main reason fire is important for our evolution is that it made it easier to eat vegetables including starchy vegetables, try eating 5 cooked carrots - super easy))
The other strong muscles are the two Masseter muscles (one is deeper than the other, so one is called Deep Masseter, the other is Superficial Masseter). These are the muscles that you can feel at along the side, near the back of the jaw. You can also see them when you bite down hard. There are other muscles involved, especially to cause side to side movement, however, these 2 are the easiest seen on yourself in the mirror.
The Meninges
In the cranium we can see that the brain has a casing round it. This casing protects the brain from the skull. This is the Meninges
The brain is protected from rubbing against the bone of the skull by the Meninges.
All animals have a meninges. Mammals have a meninges consisting of 3 layers (from the skull down):
Dura mater
Arachnoid mater
Pia mater.
The dura mater is the tough bit next to the skull
The arachnoid mater is called that because the connective tissue here makes a mesh that resembles a spider
Between the arachnoid mater and the pia mater is a space
This space is called the Subarachnoid space. This space is full of fluid. This fluid cushions the brain. Moreover, the brain floats in this fluid. This is the cerebral spinal fluid.
All animals have cerebral spinal fluid. Your brain and spine contain about 125mL or Cerebral spinal fluid. This is roughly half a cup of it.
The CSF is made in the cerebral ventricles
It is this fluid that is tested if it is suspected that you have meningitis
Meningitis is an inflammation of the meninges.
It can cause swelling and pressure on the brain.
It can be caused by several different bacteria, virus and a protozoa.
The symptoms include
Headache
Stiff neck
Fever
Rash
20% of cases of meningitis ends up with a permanent disability such as deafness, brain damage, loss of limbs
It needs to be quickly diagnosed and treated with very large doses of antibiotics (if bacterial)
Meningitis is often fatal, see here for a recent tragedy - https://www.nzherald.co.nz/nz/university-of-canterbury-student-21-dies-from-meningococcal-disease/4P6Z2B6FHSHO2EVHHEDVVJZU6Q/
Those most commonly diagnosed with Meningitis are in their teens and early twenties
Also see here: https://www.nzherald.co.nz/nz/six-meningitis-cases-in-auckland-since-october-20/AAHEEUG5K7BDAM6NVM65NXIK5A/
Cerebral Ventricles
The brain also contains two hollow spaces that provide strength to the brain - these are the cerebral ventricles.
These are where the Cerebral spinal fluid is created and they are full of the cerebral spinal fluid.
Remember that the total quantity of CSF in your body is about 125mL or They are shown in the 3 images below. Here half of the brain (a hemisphere) has been removed, along with some of the skull, allowing you to see the size of the ventricles.
Nasal Cavity
Paranasal Sinuses
This is an image above is taken inside the right nostril. We are in the nasal cavity.
Notice the tiny blue highlight on the wall - this is the entrance to a cave
The cave is inside your front cheek bone. It is called the Maxillary Paranasal Sinus
Maxillary - because it is in the maxilla. Paranasal - because it is next to the nasal cavity. Sinus - because it is a cave
In the images above and below, I have highlighted the paranasal sinuses. You can see that they are actually quite large.
The nasal cavity is connected to 4 paranasal sinuses.
Sinuses themselves are cavities. However, as they are hidden away, we barely even know they are there. But they are, they are quite large and they are important, we just haven't quite figured out what they do.
All animals have sinuses. it is quite interesting to watch the lecture on the Human surgeon helping the vets with a gorilla who has a disease in his sinus
Above here we see a series of small chambers. These are the Sinuses. These are air filled chambers. They are found in all animals. The sinuses contain mucus and small hairs, air form the nasal cavity freely moves into and out of the sinuses. Mucus and small hairs (cilia) push any dust that enters, back into the nasal cavity. The purpose of the sinus is not entirely known. However, the play roles in all of the following: Brain protection - in a head impact, the can absorb some of the impact. Immunological functions. Mucus production. Heat inhaled air. And, to reduce the weight of the skull. Its main function may be to help with diffusion of gasses for olfaction. As smells move into the nasal cavity some of the vapor or smell moves into the sinuses, they then slowly diffuse back out again into the nasal cavity - allowing the olfactory bulb to spend more time with an odor to determine what it is etc. This may also explain why when you want to smell something nice, like a rose, you breath deeply through your nose - now the odor that gets into the lungs is undetectable, also when you exhale, you don't re-smell the odor due to the diffusion of the odor into mucus in the lungs. However, often when we smell deeply of something nice, we hold our breath. This would give time for diffusion of the particles from the nose to the olfactory bulb and into and out of the sinuses, back into the nasal cavity and across the olfactory bulb
As a side note, they effect how your voice sounds, as the sound waves move through the nasal cavity and into the sinuses while talking. This contributes to the change in tone when you have a cold. There effect on your voice is more coincidental
Turbinates / Concha
Like lots of things, there are two names for the same thing.
Inside you nose are Turninates, I mean, Concha, or do I?
They are the same things - big flat curved things that take up space in the nasal cavity, warming, moistening, and cleaning air before it goes to your lungs
When you look into the nose with a endoscope, you can see these fleshy protrusions - they are also seen in the skull. You can see them in the nasal cavity above
These increase the surface area in the nasal cavity, whilst also decreasing the volume - preventing your siblings from sucking peas up into their nasal cavity. They are sensitive - preventing you from putting your finger too far up your nose when picking. This sensitivity causes you to sneeze if something is in the nasal cavity that shouldn't be there.
They serve to ensure that the air going down to your lungs is clean.
These are pinky red as they contain a lot of blood vessels. The surface of them contains cells that create mucus (snot) and their surface are coated in microscopic hairs called cilia. The cilia and mucus trap any dust and bacteria and move it towards the front of the nose to be blown out as snot (or picked at as dry snot). So the big hairs you see in your nose are only part of the nostrils. Once inside the cavity the hairs are different and are only the microscopic cilia.
So, the functions of the turbinates:
Remove dirt, dust, microbes from the air - so the air is clean
Warm the air - so it is warm in the lungs during winter
Moisten the air - as it is moist in the lungs even in a desert summer
These turbinates are typically not the source of a nose bleed. Rather the Nasal septum is
The nasal septum separates the nasal cavity into two halves or sides. The benefits of this are:
Location of direction of smell in for some animals
Maintains air flow if one cavity is blocked
Eustachian Tube
Estuation tube - auditory tube - pharyngotympanic tube = all the same 3.5cm long tube connecting the ear to the nasal cavity.
The purpose of the Eustachian tube is to allow the ear drum to move. So the ear drum, aka the Tympanic Membrane, moves when sound waves in the air hit it. It needs nothing behind it so that it can move.
Much like a car, if all the doors and windows are closed, and your friend hops out and closes their car it is quite. However, if your door or window is open, then when your friend closes the door with the same amount of force, the door will slam louder. This is because, when all the other doors and windows are closed you create an air cushion and this stops the door from slamming. If a window or door is also open, then there is no air cushion, so the door slams.
In your are, you know when this is a bit blocked, when your ears pop = this is air moving through the Eustachian tube but getting blocked by mucus as it is a very thin tube
Olfactory bulb
The yellow are nerves. In the image to the side, some of the skull and the brain have been removed so that you can see the olfactory bulb. In this image it is yellow and is underneath the purple brain. You'll notice that it is sending tiny 'shoots' through the skull. These shoots are the filaments of the olfactory nerve and are located on the olfactory bulb
The olfactory bulb is the sense organ for smell 'olfaction'
The bulb sends nerve fibers through a part of the skull that has lots of holes.
In the second image, part of the skull has been removed and you can see the part that has lots of tiny holes - in this image the holes look like little black dots.
These dots are in a bone called the ethmoid bone, and the part of the bone that has the holes is called the Cribriform plate
In the final image, you are looking up through the nasal cavity. At the top you can see the little yellow nerves hanging down - these are the olfactory receptors - nerve cells that attach to the olfactory bulb
Usually the olfactory receptors are coated in a layer of mucus. The olfactory mucosa.
These nerves are the fastest way for things to get to the brain - this is why cocaine is snorted
In normal operation, the smell of something is inhaled, some of the vapor is directed directly across olfactory mucosa
When you eat, some of the vapor from the food diffuses from the oral cavity, into the nasal cavity and then onto the olfactory mucosa where it triggers the olfactory receptors - they send an electrical signal to the olfactory bulb, the pattern of electrical signal tells us how something smells
This is also why, when you have a cold, your food doesn't taste as nice, as the high levels of mucus reduce the amount of vapor arriving at the nerve endings
Smelling food, even when you are not eating it, causes your salivary glands to increase production of saliva
Tonsils
As air and food contain bacteria the bacteria will travel down to your lungs and the stomach. 99.9% of this bacteria is harmless environmental bacteria
The bacteria that reaches the stomach is destroyed by the stomach acid - this is a major reason for the stomach acid
The bacteria that reaches the lungs is trapped in the mucus and moved back out as part of phlegm
Occasionally a bad bacteria, a pathogen will enter
To warn the immune system that a pathogen has entered, an early warning system has evolved in all mammals
The early warning system is the tonsils. These are located in the back of the mouth, nose and throat to send out the alert should a pathogen enter
The nasopharyngeal tonsil - also known as nasopharynx - but also known as the Adenoid, is located at the back of the nose.
The turbinates (aka conchas) direct the warmed and moistened and clean air, aganst the adenoid before it travels down the throat into the trachea. Any bacteria that gets past the hair, mucus and cilia of the nose can land on and settle on the the mucus of the adenoid. The immune system will check the bacteria and eat it (phagocytose) it will then take any antigens from the bacteria back to the rest of the immune system, antibody production against that antigen can then begin (if it is a new pathogen) or ramped up (if it is a pathogenic bacteria that the body has seen before.
Because of the location of the adenoid and its ability to capture pathogens for early detection, it is a location where pathogens will start to multiply first during the initial illness. Because of this there is large numbers of a respiratory pathogen when a person is sick. Because of this, this is the best place to swab when a person is sick with a respiratory illness.
The pathogen will not be only in the adenoid but also in the mucus that leaks from the adenoid. So, you don't have to swab the adenoid directly, rather you can swab just underneath it. This is the reason that we do the Nasopharyngeal Swab for COVID. It is the location in an infected person where there is the highest density of viral particles (total particles will be higher in the lungs). Because of this, if you swab here you are more likely to pick up some of the viral particles than any other method.
For eaten food and air inhaled through the mouth, you have the Palatine tonsils. These are sometimes just called The Tonsils. you can see them in the mirror if you open up your mouth, and look at the back of your mouth. They slightly further back than the Uvula (the thing that hangs down) and are on the sides of the throat.
Oral Cavity
Soft palate and Uvula
The uvula is the dangly thing you can see at the back of your mouth
The Uvula is attached to the soft palate. The job of the Uvula, along with the soft palate, is to close off the nasal cavity when you swallow.
At the back of the bottom of the nasal cavity and the top of the oral cavity, you have the soft palate. This follows on from the hard palate and is not seen on the skull as it is made of connective tissue and muscle.
The purpose of the soft palate is to stop food from entering the nasal cavity when swallowing. When you swallow the soft palate moves upwards closing off the nasal cavity.
The soft palate is also involved in speech and breathing. You can intentionally use your soft palate. Try breathing just through your nose - in this instance the soft palate has lowered and is closing off the oral cavity. Now try, breathing through just your mouth - in this case, the soft palate has risen and is closing off the nasal cavity. Now, try breathing though both the nose and the mouth at the same time - you can have to concentrate of the position of the soft palate to achieve this
The soft palate is also involved in snoring
Uvula is the part of the soft palate that hangs down. It is also involved in closing off the nasal cavity during swallowing. As it is part of the soft palate, it rises as the soft palate rises and closes off the nasal cavity. This happens during swallowing, it also closes during vomiting.
It can also close off the throat and nasal cavity if it is touched, and induce the gag reflex. This prevents unwanted objects from entering the throat. This mechanism ensures that food chucks that are swallowed are not too large for swallowing and prevents objects from becoming trapped in the throat and preventing breathing.
As you can see in the diagram, part of the issue with snoring is gravity, instead of waking up a snoring person, just turn them onto their side. If the snoring is really bad and the person themselves is having trouble sleeping, then more of the soft palate and tongue is involved. This is a problem as it can effect the persons overall health so they may need to see a doctor and even a surgeon
Hard palate
The top of the mouth has bone, this is the Hard Palate of the Maxilla.
This separates the mouth and the nasal cavity. This allows animals to chew and breath at the same time.
Often when eating, animals will push their food against the hard palate. This is seen in humans as we form the bolus.
Tongue and Taste
Should you eat it?
Your tongue will tell you
If something is rotten, or just not for human consumption - your tongue will usually tell you - because what ever it is, it'll taste gross
If something is for eating, then it will taste nice
The more sugar that thing has, like fruit, the nicer it will taste and the more you'll want to eat - if diabetes was not a thing, just how much chocolate and ice-cream would you eat?
You tongue has taste receptors - or taste buds - these look like tiny onions. They tell your brain how something tastes.
The roughness of your tongue is due to the papillae - these are tiny bumps. A single papillae can have several thousand taste buds
On certain areas of the tongue, the receptors are more sensitive for different flavors - sweet, umani (savory, meaty), salty, sour (can indicate that the food is off), bitter (can indicate that the substance is toxic)
Lips
The lips are extremely sensitive
If a eye lash falls onto your arm, you wont feel it
If an eye lash is on your lip - you will feel it
The lips are super sensitive to catch out anything in or on your food that you should not eat. They are not quite as sensitive as your tongue - a hair on your tongue feels larger than it does on your lips - you can check this with a pencil - close your eyes and rub the sharp point of a pencil across the tip of your tongue and then your lips - on which one does the end of the pencil feel larger?
The lips on humans are easily visible. Other animals, like dogs, also have lips but they are not so visible. This is due to the use of the lips in communication - both verbal and non-verbal - what is a smile?
The amount of neurons of the brain dedicated to an area has been mapped. This mapping and the unique human image, is called the Homunculus.
You can see in the image above that the lips have a very large area of the sensory area of the brain - due to how sensitive they are. They also have a large area of the motor (muscles) part of the brain, due to the many different movements that they can make (think talking and non-verbal expressions - smiling, pouting.
Interestingly, humans also uses these sensitive and movable things in a process called kissing . :-/
Epiglottis
The image above shows the Epiglottis - most of the time it is 'up' like a trap door in the floor
It is pulled downwards to close off the trachea.
Its job is to keep air flowing into the trachea, and to stop food from entering the trachea. Thus any food will move into the closed esophagus and be pushed down to the stomach
In the image to the side you can see the nasal cavity and oral cavity. They connect in the throat. Technically, the Oropharynx. (The 3 parts of the pharynx, or throat, are the nasopharynx - the part of the pharynx where the nasal cavity connects. The part of the pharynx where the oral cavity connects. And then the laryngopharynx - the bit where the trap door operates)
In the throat the air is directed down the trachea, because the epiglottis is usually open and the trachea is an open pipe, whereas the esophagus is a closed muscular tube.
The next image down is taken from the back of the tongue, looking down the throat. You can see the pale oval - this is the esophagus - you can't see down it as it is closed. You can see the vocal cords and down into the always open trachea - this is only possible because the epiglottis is up, you can just see it at the bottom of the tongue in this image.
Because the epiglottis is before the voice box (larynx), the vocal cords stay clean while you eat and swallow.
Stuff only goes into the esophagus if the epiglottis closes off the trachea and then the esophagus will cause muscular contractions that will push whatever is in the throat, down to the stomach. You can feel this when you make a fake burp - that is when you intentionally swallow air - try it, you can feel the muscles moving the soft palate to close the nasal cavity off, muscles moving the epiglottis to close off the trachea, and then the muscles of the esophagus pushing the swallowed air down the closed pipe (if it wasn't a closed pipe, then the air would simply escape out and it wouldn't be possible to make a fake burp).
When you swallow, the tongue moves to food to the back of the oral cavity, the soft palate closes, the epiglottis closes, and the back of the tongue pushes the food down the throat it passes the closed epiglottis and moves into the esophagus.
The esophagus is a closed pipe (otherwise you'd have really bad breath, and your stomach contents would go to the back of your throat every time you laid down, and would pour out every time you did a handstand). This closed pipe is wrapped in muscle. This muscle pushes the food down to the stomach, like squeezing frosting out of a pipette bag. This pushing is strong enough that you can eat lying down, you can eat in the space station with zero gravity, and you can also eat while doing a hand-stand
The esophagus sits behind the trachea. The trachea, which is an open pipe with a lid (the epiglottis). The trachea is held open by semi-circles of cartilage. You can feel this cartilage if you rub just under your voice-box. The voice-box, or larynx, is at the top of the trachea and is just below the epiglottis. When the epiglottis closes during swallowing, it protects the lungs, trachea and larynx from food particles.
The trachea is very sensitive and should anything enter it, you will cough. Because the shape of the trachea is circular, it is very important that toddlers and babies are not feed anything circular in shape as it can completely block the trachea, making it hard to suck the air in for a cough, and massively increasing the risk of death. When feeding babies you should cut anything circular (cherry tomatoes and grapes) into long shapes, or just not feed them grapes or cherry tomatoes).
Should a person not be able to cough out something then hard banging on the chest can help. Go hard, go up, you are trying to create a cough, a wave of air that will push out the food, so do not go gentle - a bruise is better than death. The sign that someone is chocking is that they will grab their own throat, you can also tell from their facial expression. Some times people die in the bathrooms of restaurants due to embarrassment. If you don't know what to do to help your friend, call out help chocking. If you are alone, dial 111 and just leave the phone emergency call centers will send a first responder.