Getting down to the bare bones of it all...

WELLLLLL today (as I was at a loss of where to go to next) we're going to hit some of the basics (well structurally that is). Today's discussion is the skull.

As I'm sure you know (in humans that is) the skull is made up of many bones. Well, it is the same for animals. The rough anatomical regions of the skull are the face, mandible and cranium (or "brain box", though that term sounds somewhat unprofessional at the best of times).

The skull is formed from two kinds of bones (embryologically speaking). The bones of the skull are endochondral bones (from cartilaginous structures and are usually unpaired) and intramembronous (From mesenchyme of the embryo (I'm sure I'll get around to discussing what that is at one point but basically it's a type of embryological tissue). The intramembranous bones are usually paired and are usually plate like.

Each of the bones shown in the picture of the cat skull are paired (each have a partner of the same shape on the opposite side of the skull).

Now bones, though solid also have quite a few holes in them (to allow for the passage of different nerves and sometimes blood supply). Not shown here but usually found in the maxilla near the orbit (eye socket) is a hole called the infraorbital foramen. Through this hole a nerve leaves the skull to innervate the upper lip and the whiskers of the animal.

Also not shown here but perhaps in another image...

Ah, here we are.. in the mandible there are some more holes. The ones shown here (the mental foramen) is where the mental nerve (a branch of the inferior alveolar nerve of Cranial Nerve V, The trigeminal nerve... sorry for that mouthful) leaves the skull. It enters the mandible (or lower jaw) on the inside of the mandibular body (about opposite and just below where the marker is pointing to the masseteric fossa). Nerves, when running through the bone do not only innervate the tissues, but also gives some sensation within the bone. This nerve here, the inferior alveolar nerve, sensitizes the lower teeth. It's partner the maxillary alveolar nerve sensitizes the upper teeth (this is why tooth
infections or chips can hurt!).

The bones of the palate (hard palate which underlies the softer tissue you can feel with your tongue (in your mouth that is) are also part of the skull. There are all sorts of bones in the head as I'm sure you can now appreciate.  Issues that can happen in this area is a failure of the secondary palate to close (the bony palate) it is called an oronasal fistula or cleft palate and leaves an open connection to the nasal cavity from the roof of the mouth. This is usually first picked up in puppies or kittens as milk comes out of their noses when they try to feed. Cleft palate is both serious (great risk of them breathing in milk) and hard to fix. Surgery here is the only option though ay not always be successful. If the primary palate fails to close it is called a cleft lip (which is more cosmetic than damaging though there is a possible increase in nasal infections). Both of these need a surgical fix.


Also visible on this skull is the incisive foramen (or palatine fissure). This is where the vomeronasal organ (or Jacobson's organ) sits. That gland is a really neat sensory gland (you've probably seen horses flip their lips upwards and make a funny face, this is called Flehmen and it's them utilizing their vomeronasal organs to scent pheromones or something new). Usually this is described in stallions "smelling" a mare in heat but both sexes do this when they smell something new, or something interesting. Flehmen forces air into the nasal cavity through slits in the hard palate forcing it through the vomeronasal organ.
Other animals (ie cats) have this ability as well.



The back of the skull also comes into play. It's where the spinal cord leave the brain to be covered with, well, spine. It too has some clinical implications. Most recently issues with the bones around the foramen magnum (big hole where the spinal cord leaves through) have been implicated in a disease recognized in the dog breed the Cavalier King Charles Spaniel.  In this breed, due to selective breeding to create a smaller dome shaped head the occipital bone is too reduced and compresses the cerebellum against the foramen magnum, as a result the trapped fluid creates lesions in the brain, neuralgia (nerve pain) and can result in the dog being in a great amount of pain. The disease itself is called syringomyelia.

There are also small little bones (associated with the skull.. well sort of) in the area of the throat. This is called the hyoid apparatus and is what allows great movement of the tongue (important to vocalization... especially in human language). It's a good idea to know both what this is and where it is so that when you radiograph a dog (or cat) you don't go "oh look at the size of that bone they've swallowed". Or alternately... it's not a great idea to stick your finger down it's throat and try to pull it out ("that's not a stick, that's suppose to be there!")

Finally.... Head shapes.. Just like ears they come in all shapes and sizes. In dogs and cats they are most commonly described as Dolicocephalic (long pointy noses like greyhounds and siamese cats), Mesocephalic (like the german shepherd and the common domestic shorthair cat) and Brachycephalic (bulldogs and persian cats). As with everything else, there can be clinical implications here too.

Brachycephalic dogs (those cute squishy faced things) tend to have multiple problems associated with the reduced skull shape. They can have breathing difficulties, popping out eyes, bad teeth and bad skin. Essentially, those skin folds are not due to excess or "just plain wrinkly skin" but due to such a reduced skull shape. They have the regular amount of skin for a dog, just not enough skull to go underneath it. They have such issues as stenotic nares (closed up nostrils) because of too much cartilage or soft tissue in this area for the underlying skeleton and therefore have trouble breathing through their noses. They have trouble breathing period due to an overlong soft palate (again for the size of their skull) which then overlies the larynx causing them to make that snorting snuffling nose as they try to breathe properly. They have issues with their bite (mallocusion) an extreme underbite that usually results in an increase in incidences of dental disease. The have popping out eyes (exopthalmic eyes) due to the reduced bone around the eye socket that can then lead to increased ulceration or even proptosis (eyes popping out!). and also they tend to have an increased risk of skin disease due to all those wrinkles trapping dirt and bacteria (definitely have to wash these little faces at night).

Riiiight, so it may not be a dog ... 

but I felt that Pacman deserved at least a nod in the reduced skeleton department

So in conclusion, there is quite a lot going on in the head and it's only going to get busier as you add on those layers (muscle, nerves and other tissues). But that said, going down to the bare bones of it all is a good way to start.