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handout: morphology of the skull

the head is very complex. we'll talk about some aspects now, someone else 
will discuss teeth and jaw, pharynx, etc.

the vertebrate skull is one of the first things that appeared 500 million 
yrs ago. another word for vertebrates is "craniates" because the skull is 
fundamental to vertebrates, as is the structure of the brain and the 
paired cranial nerves which are extremely ancient structures. initially 
there were 10 pairs of cranial nerves, then later the amniotes (reptiles, 
birds, mammals) incorporated 2 additional nerves from the neck bringing 
the total for these animals to 12 cranial nerve pairs.

shark dissection of yore is a delightful dissection because it is easy to 
dissect the cartilage of the skull area and see the brain and cranial 
nerves - 10 pairs. so there's a long evolutionary history behind the 
structures we will look at.

one thing about the head being so complex and dense and compact is that 
all the body systems are represented in the head, coming together in a 
very small area. skull=skeletal system, muscles=muscle system; 
mouth,teeth,lips,tongue  =digestive system, respiratory system; nervous 
system;, vascular system;, reproductive system=vomeronasal organ: a small 
tubular organ located in the floor of the nasal cavity which communicates 
to the outside world via a structure on the roof of the mouth, rostrally 
located - the palatine fissures of the hard palate, which in life are 
covered by the mucosa inside the mouth, and there is a diamond shaped 
prominence: the incisive papilla, located just caudally to the incisors. 
the ducts of the vomeronasal organ open onto the incisive papilla. this 
gland is an accessory chemoreceptor/olfactory organ which is involved in 
a biochemical assay of urine. many mammals engage in urine sniffling 
behavior - horse nuzzles mare to stimulate urination, then takes urine in 
and performs "flehmen", sticking head up in the air, pushing nose up - 
that silly face they make :). cats, lions, giraffes do this. males do it 
more than females though some species females do do it. some dominant 
mares will do it. another reproductive organ in the head has to do with 
how to tell the sheep from the goats....there is a gland on the head of 
many ruminants and this is the preorbital gland. we notice this gland as 
a slit in front of the eye, prominent in sheep. in bighorn sheep has 
signifcant behavioral significance - involved in interaction between 
males. the bighorn sheep have big huge horns and they do fight for mating 
rights. but big rams do not fight with little rams because small, 
younger, weaker rams are submissive to the dominant rams. dominant rams 
strut around with their heads up assessing eachother's horns etc.the 
lesser males don't want to fight, they go up with their heads low, and 
rub their faces on the dominant male's face. so the scent of the dominant 
ram is conferred onto the lesser ram. all the females and subordinate 
males carry the scent of the dominant ram. this is in sheep. the ram who 
won't take scent gets in a fight and the loser leaves. now, goats do not 
have this preorbital gland. they have an anal gland. subordinate behavior 
in goats involves the subordinate males rubbing face on the anus of the 
dominant male - kissing his ass, so to speak, according to the inimitable 
dr dodson. female goats don't kiss eachother's asses, and females don't 
kiss male asses, but the subordinate males kiss the ass of the dominant 
male goat.

in terms of the structure of the skull, it's kind of a box with lots of 
spaces and prominent external features. 

major openings rostrally the naris, the orbit in the middle, and on the 
occipital/caudal surface is the foramen magnum, which is dorsal to the 
occipital condyles. the lateral surfaces or the caudal skull are the 
nuchal crests. there are jugular processes lateral to the occipital 
condyles, and there are bullae around the external auditory meatus, and 
caudal to the bulla is the mastoid process. there is also a nerve 
forament there, the stylomastoid foramen - between jugular process and 
external auditory meatus - through which cnVII the facial nerve runs 
through.

the infraorbital canal is in front of the orbit. the opening to the 
actual eye socket is ccalled the maxillary foramen, and through that 
foramen goes the infraorbital nerve artery and vein which are 
continuations of the maxillary n,a,v, which go out into the face passing 
underneath the eyeball.

also laterally on the skull is the zygomatic arch. take a look at a 
diagram to see where these things all go, ok?

will look at cranial nerves in relation to skull foramina later.

a conceptual step forward: looking at the spaces within the skull.

if we look at a sagittal section we can see spaces: the cranial cavity 
and the nasal cavity, for the brain and the respiratory system, and the 
oral cavity for the digestive system. now, in dogs, and some other 
animals there isn't too much covering the brain. in say, an elephant, 
there is proportionally MUCH more cancellous bone covering the brain. 
also sheep, cattle. but dogs, horses, people, all that separates brain 
from outside world is relatively thin bony shield. another thing to see 
is the nostrils are located rostrally and in species with long muzzles 
there is a long nasal cavity protecting the brain. in OUR species, brain 
is right behind the nose, so sometimes we can inhale medications or other 
things through the nose and they get right to the brain.
(go, dr d!)

the nasal bones bound the nasal cavity dorsally, and the hard palate 
bounds it ventrally. the hard palate is formed by the maxilla and the 
palatine bone. the soft palate is a soft structure suspended between the 
palatine bones and the pterygoid bones. in humans you can see the free 
edge of the soft palate has a little fancy thing hanging down, and that's 
the uvula, and it is quite simply a drip structure allowing snot to drain 
from nasal cavity into the pharynx, it's a drip point, that's all. if you 
look in human mouth you see a black hole w/uvula in midline and on either 
side are the palatoglossal arches. those arches run between the palate 
and the glossus (tongue).

a cross section of the nasal cavity reveals the nasal septum made of 
cartilage and numerous scrolls of thin lacy bone, aka conchae- the dorsal 
concha and ventral concha, and they are covered w/epithelium and have a 
rich blood supply and lots of glands making mucous and serous secretions. 
there are four passages or meati (meatuses?). there is a dorsal meatus 
between the dorsal concha and top of nasal cavity. there is a middle 
meatus between the conchae, and there is a ventral meatus between the 
ventral meatus and the ventral wall of the cavity. also the common meatus 
is between the medial surfaces of the conchae and the septum itself. most 
airflow goes through ventral meatus, but also through other regions. the 
most anterior part of the brain has an olfactory function. olfactory 
nerves come through and are situated in the caudal region of the nasal 
cavity in the ethmoturbinals, which  pass through the ethmoid bone. the 
sense of smell is localized to the ethmoturbinals.

at caudal border of nasal cavity, at end of turbinals, beginning of clear 
passageway, situated at junction between hard and soft palate, is the 
choana aka the internal nostril, and this is where air leaves nasal 
cavity and enters the nasal pharynx. the soft palate separates the 
nasopharynx from the oropharyx below it. in a way the choana isn't an 
actual STRUCTURE per se but rather it is a descriptive term designating 
the end of the nasal cavity and the beginning of nasopharynx. so air 
passes from ventral meatus into the nasopharynx in the region of the 
choana. just realize that that's the name for that area.

ok. the oral cavity is bounded dorsallyl by hard palate, and ventrally by 
bottom of jaw, laterally by cheeks. oral cavity is where voluntary 
chewing takes place.

the tongue [diagram] in sagital section runs between the rostral mandible 
(chin region - genio) and the basihyoid bone. the GENIOGLOSSUS muscle 
runs from chin to tongue, is the muscle that sticks tongue out, extends 
tongue. the HYOGLOSSUS runs from back of tongue to basihyoid bone. during 
swallowing the geniohyoideus contracts, pulling the laryngeal apparatus 
toward the chin, and this is opposed by the sternohyoideus muscle which 
pulls the larynx back down again.

hyoglossus btw pulls tongue back into mouth. styloglossus runs from 
tongue up and pulls tongue up.

see pictures/drawings

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