lecture note - skull and visceral skeleton

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  • 8/12/2019 Lecture Note - skull and visceral skeleton

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    1Lecture Notes: Animal Morphology & Anatomy by Ms. Manilyn C. Lopez

    PARTIDO STATE UNIVERSITYCOLLEGE OF ARTS AND SCIENCES

    Goa, Camarines Sur

    LECTURE NOTES ON ANIMAL MORPHOLOGY AND ANATOMY

    Topic: THE SKULL AND VISCERAL SKELETON

    - The vertebrate skull consists of 3 parts: neurocranium, dermatocranium, and splanchnocranium.

    - Classification:

    Neurocranium

    Dermatocranium

    Splanchnocranium: visceral skeleton

    Palatoquadrate cartilage and replacement bones

    Meckel's cartilage and replacement bone

    Skeleton of the branchial arches

    3. Neurocranium: primary braincase that (1) protects brain, (2) arises as cartilage, which (3) is replaced by bone(except in cartilaginous fish). Similar development occurs in all vertebrates.

    Formation: arises from paired prechordal and parachordal cartilage beneath the brain. Cartilage also developsaround the olfactory and otic capsules (and may develop around the eye). As development progresses, parachordacartilage unite to form the basal plate and unites with the otic capsules, while prechordal cartilage forms theethmoid plate and unites with the olfactory cartilage.

    Further development involves formation of walls and, in lower vertbrates, a cartilaginous roof (tectum). Fenestraand foramina accomodate blood vessels and cranial nerves.

    Parachordal cartilage arises from mesenchyme of the lateral plate mesoderm, while prechordal cartilage arisesfrom neural crest ectoderm.

    Cartilaginous neurocrania in adult vertebrates:

    Cyclostomes: individual components remain more or less independent throughout life.

    Cartilaginous fish: components unite to form an adult chondrocranium which encloses the brain.

    Lower bony fish: a cartilaginous neurocranium persists in adult chondrosteans and holosteans.However, this cartilage is overlain with dermal bone.

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    Lecture Notes: Animal Morphology & Anatomy by Ms. Manilyn C. Lopez

    Ossification centers in the neurocranium: multiple, separate areas where endochondral ossification occurs inbony vertebrates. Major regional groups include:

    Occipital centers - as many as four areas around the foramen magnum (2 exoccipitals, basioccipital,and supraoccipital)

    Sphenoid centers - under the midbrain and pituitary gland include the basisphenoid and presphenoid.These together with lateral sphenoid elements (orbitosphenoid, pleurosphenoid) form

    the adult sphenoid bone.

    Ethmoid centers - anterior to sphenoid. Form the ethmoid plate and olfactory capsules. Ethmoid tendsto remain cartilaginous even in mammals. Cribiform plate allows passage of olfactory nerves to theolfactory epithelium.

    Otic centers - several bones form here and may be replaced or fuse together, eg. theprootics, opisthotic, and epiotics unite to form the petrosal bone which thenfuses with the squamosal bone to form a temporal bone.

    4. Dermatocranium: these are the membrane bones of the skull and may have originated in the bony dermal armor

    of the ostracoderms.

    In modern vertebrates including man, membrane bones of the head originate from subdermal mesenchyme ofneural crest and lateral plate mesoderm rather than dermal mesenchyme.

    Basic structural elements include roofing bones of the neurocranium, marginal bones of the upper jaw, bones ofthe primary palate, and opercular bones.

    Roofing bones: paired and unpaired bones that in crossopterygians extend down the mid-dorsal line fromthe nares to the occiput. Include nasals, frontals, parietals, and dermoccipitals.

    Around the orbit of the eye were the lacrimal, prefrontal, postfrontal, postorbital, and jugal.

    At the posterior angle of the skull were intertemporal, supratemporal, tabular, squamosal, and quadrojugal bones.

    Marginal bones (upper jaw): palatoquadrate cartilage becomes ensheathed by premaxillae and maxillae.

    Primary palatal bones: form the roof of the oropharyngeal cavity.

    In crossopterygeans and primative tetrapods, these membrane bones included a parasphenoidand paired volmers, palatines, pterygoids, and ectopterygoids.

    The primary palate is still present in modern tetrapods as the roof of the nasal cavity.The oral and nasal cavities are divided by a secondary palate.

    Opercular bones: protect the operculum, a flap of tissue from the hyoid arch. When present, opercular bonesare dermal. Absent in tetrapods. Major bones include the opercular, preoperculars, suboperculars,and interoperculars.

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    Lecture Notes: Animal Morphology & Anatomy by Ms. Manilyn C. Lopez

    Skull and Visceral Skeleton II

    Neurocranial-Dermatocranial complex of bony fish:

    1. Chondrosteans: this superorder includes the spoonbill and paddlefish. In these animals the neurocraniumremains cartilaginous throughout life. Traces of ossification occur in the otic capsules and in that portion of the sphenoidthat contributes to the orbit of the eye. Dermal bones may obscure the neurocranium.

    2. Holosteans: Bowfin and garfish have skulls similar to the chondrosteans with the neurocrania remainingmostly cartilaginous. Most obvious are the dermal bones which are sculptured to correspond to the underside of thedermis.

    3. Teleosts: modern teleosts show skulls which are highly specialized and diverse, corresponding to the diversefeeding habits of this group. Bones associated with the jaws of a typical teleost include the maxillae, premaxillae,dentary, articular, quadrate and symplectic. Common roofing bones are the frontal, parietal, supraoccipital, andposttemporal.

    4. Dipnoans: lungfish have similarities to all of the previous groups and yet show obvious differences. Typically the

    dipnoan skull is more conservative. The dermatocranium has evolved into only a few bony plates while the neurocraniumremains cartilaginous.

    The Neurocranial-Dermatocranial complex of modern tetrapods

    1. Amphibians: neurocranium incomplete dorsally and largely cartilaginous. Articulating with the otic capsule isthe columella which conducts sound from the eardrum to the capsule (comes from the hyomandibula). Dermatocraniumlacks the bones that surround the orbit except for the lacrimal and prefrontal. Temporal bones are also missing or reduced.In the otic region, only the squamosal and quadrojugal remain. The primary palate has been altered to accomodate theeyes.

    2. Reptiles: living orders show a well ossified neurocranium with a single occipital condyle and a larger number ofmembrane bones than amphibians. Many possess a parietal foramen, temporal fossae, and a complete secondary palate.

    Temporal fossae: openings in the temporal region of amniotes bounded by one or more bony arches. Early stem reptileshad none (anapsid), which is also the condition in modern turtles. The synapsid condition involves a temporalfossa bounded by postorbital, squamosal, and jugal bones; today this is the zygomatic arch of the mammalian skull. Thediapsid skull was characteristic of ancestral snakes and lizards. Extant snakes and lizards, have modified diapsid skulls.

    Secondary palates: appear first in reptiles as a horizontal partition that divides the oral cavity into oral andnasal passages. In crocodilians, palatal processes of the premaxillae, maxillae, palatine, and pterygoid bones meet in themidline to form a secondary palate. In mammals, the premaxillae (not in humans), maxillae, and palatine bones form the

    secondary palate.

    Cranial kinesis: independent movement of one or more parts of the neurocranial-dermatocranial complex. In the caseof lizards, the quadrate, upper jaw, orbital bones, and the parietal bone may move as a unit, independent of the braincase

    3. Birds: similar to reptilian skull with modifications for flight and feeding. Some roofing bones lost; dermal bones reduced.

    4. Mammals: here the dentary bone becomes the sole bone of the lower jaw. Neurocranium incomplete with fontanels innewborns.

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    Lecture Notes: Animal Morphology & Anatomy by Ms. Manilyn C. Lopez

    Bregmatic bones may ossify in the frontal fontanele of some species (a single bone is sometimes found inhumans). Ossification centers in the neurocranium are similar to those previously described. Dermatocranium representedby pairs of premaxillae, maxillae, jugals, nasals, lacrimals, and squamosals. Frontals, parietals, and interparietalscomplete the series.

    The Visceral Skeleton:

    The splanchnocranium is the skeleton of the pharyngeal arches in fish (jaws and gill arches) and has given rise to somevery interesting structural components in mammals.

    1. Sharks: visceral skeleton consists of cartilage in each arch as well as median basihyal and basibranchial cartilages inthe floor of the pharynx. First arch modified for feeding as the mandibular arch. Consists of the palatoquadrate andMeckel's cartilages. The second, or hyoid arch, components include the hyomandibular (dorsally) and ceratohyal (lateral)cartilage. Articulation of the palatoquadrate and Meckel's cartilages includes the hyomandibular in a movable joint.The hyomandibula is bound by ligaments to the otic capsule and thus suspends the jaws from the neurocranium: hyostylicjaw suspension. Amphystylic attachment, where the palatoquadrate is attached at several locations to the neurocranium, isseen in some ancient sharks. Autostylic attachment occurs when the palatoquadrate is fused to the neurocranium.

    2. Bony fish: embryonic cartilage is ensheathed by membrane bone.Palatoquadrate is overgrown by premaxillae and maxillae. Palatal region replaced by palatine and ectopterygoidswhile the posterior tip ossifies to form the quadrate bone. The caudal end of Meckel's cartilage forms the articularbone, while the remainder forms the dentary, surangular, and angular bones. Hyoid cartilages form symplectic, interhyals,and epihyals. Articulation of the jaw may involve the symplectic and quadrate, or symplectic, quadrate, and lower jaw.

    3. Tetrapods: modifications of visceral skeleton correspond with adaptational changes for terrestrial life. Palatoquadrateand Meckel's cartilage become ensheathed by dermal bones: premaxillae, maxillae, and palatal bones(palatoquadrate); quadrate becomes site of articulation with lower jaw in tetrapods below mammals (becomes incus inmammals); Dentary, angular, surangular, splenial, coronoids, prearticulars, and articulars form in Meckel's (articular bone

    articulates with quadrate, except mammals where it forms the malleus). A new articulation formed between the dentarybone and the squamosal, now known as the temporomandibular joint. Note that mammals have only the dentary boneforming the lower jaw.Remember the the hyomandibula becomes the stapes (columella).The remainder of the visceral skeleton contributes to the support of the larynx.