----start anat 11.18.96.b-----

Thurs 4 pm room 100 meeting for class of 2000.

TEETH AND JAWS - barbara grandstaff

BASIC INFO:
teeth are mineralized skeletal elements in or near the mouth. involved 
w/acquisition and processing of food; grooming, combat, display. always 
attached to membrane bone, not endochondral bone. can give info about age 
of animal, diet of animal, species of animal. also can identify sex of 
animal. female horse has no canines, male does. 

STRUCTURE of TOOTH:
see fig 3-15 dyce (in handout)
the simple tooth, eg dog incisor, has different tissues within it. the 
outermost tissue is ENAMEL. this covers the crown/upper part of the tooth 
and is extremely hard. it's the hardest biological material there is - 
97% mineral (hydroxyapatite). the other 3% is partly biological and 
partly water. within the enamel is a microstructure of enamel prisms 
which cross eachother to form hunter schrager bands (sp?)
beneath the enamel is the yellowish DENTINE, which covers the ENTIRE 
tooth, all the way down to the apex of the root. covering the dentine on 
the root is the cement. dentine is not nearly as mineralized - only about 
80% hydroxyapatite...
DENTINE is located INSIDE the enamel, makes up the bulk of the tooth. the 
largest volume of any tooth is dentine. there is also a microstructure to 
the dentine: dentine tubules, microscopic tubules running through dentine 
from pulp toward enamel-dentine junction. these tubules are present 
throughout the dentine. the dentine tubules of the tooth contain cellular 
processes from the odontoblasts (dentine secreting cells of the pulp) and 
these processes, because they are vital, make the dentine sensitive. so 
the enamel is insensitive, but the dentine is sensitive, and can feel 
pain, esp heat and cold.

the PULP cavity is where the neurovascular supply comes in. the pulp has 
blood vessels, nerves, CT, and odontoblasts, and fills the pulp cavity. 
odontoblasts secrete dentine and send processes into the dentine tubules. 
the pulp cavity is LINED with odontoblasts. you can't see those grossly, 
however.

outside of the root, overlying the dentine, is the CEMENT. this covers 
the entire root in all types of teeth. cement is an acellular bony tissue 
produced by osteoblasts. it is always covering the roots of teeth. in the 
high crowned teeth cement is also present on part of the crown.

the tooth fits into a SOCKET in the bone of the jaw. that's called the 
ALVEOLUS.
a tooth may have only one root or may have multiple roots (eg 
molars/cheek teeth). the teeth are held into the socket by a soft tissue 
which makes a fibrous connection beetween the alveolus and the cement - 
this is the periodontal ligament. these fibers slope down into the bottom 
of the socket to prevent the tooth from pushing against the bottom. when 
a tooth is extracted the periodontal ligament must be cut.

the final tissue is the GINGIVA or gum. this tissue overlies the bone and 
extends up around the tooth as the gingival cuff.

in mammals the tooth is attached into a closely fitting alveolus in 
membrane bones. the growth of teeth is in the handout but we won't 
discuss it. we'll hear about that next week in embryo.

another aspect of teeth. notice that there are 3 anatomical regions to 
the tooth. there is the root, which is the part within the actual socket, 
and is covered w/cementum but has no enamel. it is held in place by 
periodontal ligament. dorsally is the crown of the tooth, which is the 
part of the tooth outside the socket, covered in enamel, used in chewing. 
may be simple or complex depending on function. between these parts is 
the NECK which is beneath the end of the enamel but above the cementum. 
this is where you grab the tooth to pull it out. it's the narrowest part 
of the tooth.

directional terms:

lingual: toward the tongue
labial: toward the lips
buccal: toward the cheeks (also called "vestibular")
mesial: toward the front midline.
distal: toward the jaw hinge

one aspect of tooth anatomy that's very important is anatomy of jaw 
muscles (huh?) 
teeth are not isolated structures floating around in space except in 
paleontology where isolated teeth represent entire species. 

real world teeth are in jaws and jaws have muscles of mastication 
associated with them which are extremely important for chewing and 
eating. 

if you look at a skull from the back there are three muscles of closing 
the jaw. 

dorsally is the temporalis- from the sagittal crest to the coronoid 
process.
from the zygomatic arch to the angle of the jaw is the MASSETER.

from the base of the skull to the medial angle of the jaw and the wing of 
something is the PTERYGOIDS

temporalis: about 50% of the jaw muscle mass
masseter about 30%
pterygoids about 20%
above is for people, generalized.

for dog, temporalis about 64%, masseter about 28%, and the pterygoids are 
about 8% - no wonder it is so hard to find them.

in an herbivore, say horse or goat, temporalis only 24%, masster about 
50%, and pterygoids about 30%

in a rodent like the capybara, the temporalis is only 17%. the masseter 
is 66%, and the pterygoids are 18%. 

so this has a wide variation.
the TEMPORALIS functions by closing the jaw more or less like the 
scissors. closing jaw by rotation around the temporomandibular joint. so 
when you look at a carnivore eg cat, the closure of the jaw is very 
precise and is guided by the large canines. and the carnassial teeth 
slice past eachother like scissor blades. so the temporalis is very 
important to the carnivore. 

the MASSETER inserts down on the massenteric fossa toward the angle of 
the jaw, and is enlarged in a plant eater like a horse. in these animals 
the teeth close parallel to each other, not like scissors, so there is a 
grinding/crushing going on. in these animals the jaw hinge is much higher 
up than the tooth row. in the carnivore the hinge is level with the tooth 
row. so herbivore jaws are more like a nutcracker. this enhances the 
grinding ability of their ridgelike teeth, which are quite different from 
the bladelike feline / carnivore teeth. the horse jaw is also capable of 
lateral motion for grinding purposes, and this is needed b/c the upper 
teeth lie lateral to the lower teeth, and you need the lateral stroke for 
good grinding activity. to do this you need to use the masseter on one 
side and the pterygoid on the other, and you get lateral motion in the 
direction of the active masseter. COWS do this a lot :)

masseter fibers on a rodent go all the way to back of the nose. in the 
horse they also extend forward onto the face on the elongation of zyg 
arch - the facial crest. 

difference between generalized animal and carnivore...
HEIGHT of teeth. in carnivores the neck of the tooth is at the gumline. 
in the horse, it's way up under the maxilla, reaching far up into the 
skull, partially filling the maxillary sinus. 

LOW CROWNED TOOTH: BUNODONT- seen in carnivores

HIGH CROWNED TOOTH: HYPSODONT-seen in horses, etc. evolves in response to 
abrasive diet. grass cells contain silica bodies which abrade teeth. so 
grass eating animals evolved these very high crowned teeth. 

rodents evolved open rooted, continuously growing teeth
hypsodont teeth are characteristic of herbivores especially grazers eg 
horse, cattle.

last page of handout - clinical aspects of teeth. decay can be fatal, 
malocclusion can be fatal in rodent (tooth can grow into brain). these 
are non trivial problems. don't have time to discuss today.





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