Chapter Six: The Axial Skeleton and Skull

Vocabulary flashcards covering vertebrae, ribs, sternum, skull, and the neurocranial/visceral skeleton concepts from the notes.

Centrum

The central portion of a vertebra around which the vertebral develops; forms the main weight-bearing element and encases the notochord during development.

Neural Arch

The dorsal part of a vertebra that protects the spinal cord; together with other arches forms the Neural/Vertebral Canal.

Notochord

A flexible rod found in the embryo that provides skeletal support; in vertebrates it is surrounded by the centrum and can persist in varying degrees.

Neural/Vertebral Canal

The canal formed by successive vertebral arches that houses the spinal cord.

Hemal Arches

Ventral arches in tail vertebrae that enclose the caudal artery; also called chevron bones in many amniotes.

Chevron Bones

Common name for hemal arches in the tail of amniotes.

Transverse Processes (Diapophyses)

Lateral projections of vertebrae that articulate with ribs and provide muscle/ligament attachment.

Articulating Processes (Zygapophyses)

Processes that permit articulation between successive vertebrae; include prezygapophyses (cranial) and postzygapophyses (caudal).

Prezygapophyses

Superior articulating processes on the cranial aspect of a vertebra that connect with the vertebra cranial to it.

Postzygapophyses

Inferior articulating processes on the caudal aspect of a vertebra that connect with the vertebra caudal to it.

Parapophyses

Paired lateral processes on the centrum that articulate with the capitulum of the rib.

Hypophyses

Prominent midventral projections from the centrum; attachment sites for muscles and tendons.

Blastema

Mass of mesenchymal cells formed when sclerotome cells exit the somite and migrate to form vertebrae.

Chondroblasts

Cartilage-forming cells that differentiate in the blastema to form centrum and arches.

Perichordal Bone/Cartilage

Bone or cartilage surrounding the notochord around the developing centrum.

Chordal Cartilage

Cartilage surrounding the notochord that can remain in the centrum or ossify depending on species.

Intercentrum

An early or accessory centrum component in some vertebrae, located ventrally in the developmental pattern.

Pleurocentrum

A vertebral centrum that lies dorsolaterally; often forms with the intercentrum in certain patterns.

Diplospondyly

Condition where there are two centra per body segment, common in some fish tails.

Sclerotome

A portion of the somite that gives rise to vertebral elements via migration and ossification.

Rhachitomous

A primitive vertebra type with a hypocentrum, pleurocentrum, and neural arch surrounding the notochord.

Hypocentrum

Anterior ventral part of a rachitomous vertebra that cradles the notochord.

Pleurocentrum (again)

Dorsolateral portion of the centrum contributing to vertebral body in some patterns.

Intercentrum

An embryonic or partial body segment element that can be present with pleurocentrum.

Amphicelous Vertebrae

Vertebrae whose centra are concave on both ends, allowing articulation with adjacent vertebrae.

Procelous Vertebrae

Vertebrae with a concave cranial (anterior) surface of the centrum and a convex caudal surface.

Opisthocelous Vertebrae

Vertebrae whose caudal surface is concave, with the hypocentrum attaching to the vertebra caudal to it.

Acelous Vertebrae

Vertebrae lacking concavities on either end; intervertebral discs provide cushioning.

Atlas (C1)

The first cervical vertebra; ring-shaped with no substantial centrum; enables nodding by articulating with the skull.

Axis (C2)

Second cervical vertebra; bears the Dens/Odontoid Process that pivots the head on C1.

Dens (Odontoid Process)

The odontoid projection from C2 that articulates with C1 to permit head rotation.

Proatlas

A cartilage or cartilage-like element present in some basal amniotes; later replaced or reduced in many modern groups.

Heterocelous Vertebrae

Bird cervical vertebrae with saddle-shaped (asymmetrical) articulations allowing extensive neck mobility.

Cervical Vertebrae

Neck region vertebrae; typically highly mobile with variations across tetrapods.

Thoracic Vertebrae

Vertebrae associated with ribs and protecting thoracic viscera; provide rib articulation and support the pectoral girdle.

Lumbar Vertebrae

Vertebrae in the lower back; heavier and more robust for weight-bearing.

Sacral Vertebrae

Vertebrae that articulate with the pelvis; typically fuse to form the sacrum in many tetrapods.

Caudal Vertebrae

Vertebrae of the tail; number varies; include specialized tail structures in different lineages.

Synsacrum

In birds, the fused lumbar, sacral, and some caudal vertebrae that form a rigid pelvic region.

Pygostyle

Bird tail structure formed by the fusion of the last caudal vertebrae.

Urostyle

Anuran (frog) tail structure formed from the fusion of a perichordal cartilage during metamorphosis.

Intervertebral Disc

Cartilaginous disc between adjacent vertebrae containing a gelatinous nucleus pulposus.

Nucleus Pulposus

Central gel-like core of an intervertebral disc derived from the notochord.

Rib (Vertebrosternal)

Rib that articulates with the sternum via its costal cartilage; true rib.

Vertebrochondral Rib

Rib that attaches to the sternum indirectly via costal cartilage; false rib.

Vertebral/Vertebral Ribs (Floating)

Ribs that do not attach to the sternum; attach only to vertebrae.

Costal Cartilage

Cartilage connecting the ribs to the sternum in many tetrapods.

Gastralia

Gastral ribs in some reptiles that lie in the body wall and are not true ribs.

Sternal Ribs

Ribs that connect to the sternum via costal cartilage.

Sternum

Breastbone in tetrapods; supports limb girdles and provides attachment for chest muscles.

Carina (Bird Sternum)

Keel-like projection on the avian sternum for attachment of massive pectoral muscles.

Sternebrae

Bony segments that comprise the mammalian sternum; may fuse to form a single sternum.

Xiphoid Process

Small cartilaginous extension at the inferior end of the sternum; can ossify in some species.

Skull/Splanchnocranium

Visceral skeleton that forms jaw and gill arch structures; distinct from the neurocranium.

Neurocranium

The primary brain case; protects the brain and sense organs; includes chondrocranium and later dermal bones.

Dermatocranium

Dermal (membrane) bones that form the roof and walls of the skull, derived from dermal bone.

Chondrocranium

Cartilaginous part of the neurocranium; forms the cartilaginous brain case in early development.

Splanchnocranium (Visceral Skeleton)

Skeleton derived from pharyngeal arches; includes jaws, hyoid, and gill arch derivatives.

Palatoquadrate

Embryonic upper jaw cartilage that becomes articulating bones; part of the mandibular arch.

Meckel’s Cartilage

Embryonic lower jaw cartilage that ossifies into jaw bones in most groups.

Pharyngeal Arches

Series of arches in the embryonic head; give rise to gill arches and jaw elements.

Hyostyly

Jaw suspension pattern where the jaw is braced by the hyomandibular against the skull; common in sharks.

Amphistyly

Primitive jaw suspension where hyomandibular and palatoquadrate braces help suspend the jaw.

Autostyly

Jaw suspension where the palatoquadrate attaches directly to the neurocranium; ligaments suspend the rest.

Stapes (Columella)

Auditory ossicle derived from the hyomandibular cartilage in non-mammals.

Malleus

Auditory ossicle derived from the articular bone in mammals.

Incus

Auditory ossicle derived from the quadrate bone in mammals; part of the middle ear chain.

Quardate

Bone in the jaw joint that becomes the incus in mammals; part of the original jaw joint in nonmammals.

Articular Bone

Jaw joint bone in early tetrapods; becomes the malleus in mammals.

Auditory Ossicles

Malleus, Incus, and Stapes; bones of the middle ear that transfer sound.

Stapes Origin

Originates from the hyomandibular cartilage and forms part of the middle ear in nonmammals.

Tympanic Bone/Tympanic Bulla

Bones surrounding the tympanic membrane; part of the mammalian ear region.

Petrosal (Periotic)

Bone formed from fusions of various otic bones; part of the temporal bone in mammals.

Temporal Bone

Bone around the skull’s temporal region; formed from fusion of petrosal with others.

Fontanels

Soft spots in the neonate skull where the neurocranium has not yet ossified; allow growth during birth.

Neurocranial-Dermatocranial Complex

Integration of neurocranium and dermatocranium elements across vertebrates.

Ethmoid Plate

Midline ossification plate that forms part of the skull floor and nasal region; derived from prechordal/cartilaginous elements.

Basal Plate

Cartilaginous plate forming part of the skull floor; joins with parachordal elements.

Hypophyseal Fenestra

Opening in the cranial floor that houses the pituitary and arteries; later ossifies into foramina.

Ossification Centers (Occipital)

Centers that form basioccipital, exoccipital, and supraoccipital bones of the occipital region.

Occipital Condyles

Articulation surfaces on the skull that connect to the first vertebra; differ between mammals (two) and birds (one cerc when present).

Basisphenoid/Presphenoid/Alisphenoid

Sphenoid region bones formed by multiple ossification centers in mammals and archosaurs.

Ethmoid (Mesethmoid, Nasal Turbinates)

Ethmoid region bones including mesethmoid; contribute to nasal septum and orbital structures; includes nasal turbinates in many vertebrates.

Sphenethmoid

Single bone comprising sphenoid-ethmoid complex in some amphibians (and in some reptiles).

Otic Capsules/Prootic/Opisthotic/Epiotic

Bones surrounding the inner ear; fuse to form Petrosal/Periotic; related to the temporal region.

Sella Turcica

Bony saddle in the basisphenoid that houses the pituitary gland in many mammals.

Mesethmoid/Cribriform Plate

Ethmoid region bones forming nasal septum; cribriform plate bears olfactory foramina.

Palatal Fissure

A deep groove in the palate seen in birds and some reptiles, related to the secondary palate's development.

Secondary Palate

Horizontal bone partition separating nasal and oral cavities in many amniotes; can be incomplete in some groups.

Primary Palate

Roof of the oral cavity formed by dermal bones; in some groups remains cartilaginous.

Gular Bones/Branchiostegal Rays

Dermal bones forming part of the gill region in fishes; branchiostegal rays accompany the opercular membrane.

Opercular Bones

Dermal bones covering the gills in fishes; include opercular, preopercular, subopercular, and interopercular bones.

Dermal Skeleton

Bones that originate in the dermis (dermatocranium) rather than from endochondral ossification.

Neurocranium vs Dermatocranium

Neurocranium protects the brain; dermatocranium forms the outer skull bones from dermal origins.

Parasphenoid/ vomers/ palatines/ ectopterygoids/ pterygoids

Bones forming the primary palate and peripheral skull regions in early gnathostomes; some persist in various lineages.

Jugal (Infraorbital)

A facial bone forming part of the orbit; part of the roofing bones around the eye.

Lacrimals/ Prefrontals/ Postfrontal/ Postparietals

Roofing bones around the orbit region; variably present across tetrapods.

Premaxilla/Maxilla/Dentary/Articular/Quadrate

Jaws and jaw-support bones; in mammals the dentary dominates the lower jaw; quadrate and articular are repurposed as auditory ossicles.

Splanchnocranium in Sharks

First pharyngeal arch (mandibular) forms jaws; second arch (hyoid) supports gills and jaw suspensions; gill arches 3-7 contribute to hyoid and branchial skeleton.

Mandibular Arch/Meckel’s Cartilage

First pharyngeal arch forming upper/lower jaws in early vertebrates; Meckel’s cartilage ossifies into jaw bones in many groups.