Axial Skeleton

SKULL
Cranium: encloses and protects the fragile brain tissue and is composed of eight large, flat bones 
Facial Bones: hold the eyes in an anterior position and allow the facial muscles to show our feelings through smiles or frowns 
Frontal Bone: the frontal bone forms the forehead, the bony projections under the eyebrows, and the superior part of each eye's orbit
Parietal Bones: the paired parietal bones form most of the superior and lateral walls of the cranium (they meet in the midline of the skull at the sagittal suture and for the coronal suture where they meet at the frontal bone) 
Temporal Bones: the temporal bones lie inferior to the parietal bones; they join them at the squamous sutures
~~~ External Acoustic (Auditory) Meatus: the canal that leads to the eardrum and the middle ear
~~~ Styloid Process: a sharp, needlelike projection just inferior to the external auditory meatus ~ this serves as an attachment point for many neck muscles 
~~~ Zygomatic Process: a thin bridge of bone that joins with the cheekbone (zygomatic bone) anteriorly
~~~ Mastoid Process: a rough progression posterior and inferior to the external acoustic meatus, which is full of air cavities (mastoid sinuses); providing an attachment site for some muscles of the neck
~~~ Jugular Foramen: located at the junction of the occipital and temporal bones, this allows passage of the jugular vein, the largest vein of the head, which drains the brain
~~~ Internal Acoustic Meatus: transmits cranial nerves VII and VIII (the facial and vestibulocochlear nerves)
~~~ Carotid Canal: located anterior to the jugular foramen on the skull's inferior aspect, this canal in which the internal carotid artery runs, supplies blood to most of the brain
Occipital Bone: the most posterior bone of the cranium in which forms the floor and back wall of the skull
~~~ Foramen Magnum: large opening that surrounds the lower part of the brain and allows the spinal cord to connect with the brain
~~~ Occipital Condyles: structures that rest on the first vertebra of the spinal column
Sphenoid Bone: the butterfly shaped bone that spans the width of the skull and forms part of the floor of the cranial cavity
Sella Turcica: also called Turk's Saddle this holds the pituitary gland in place
Foramen Ovale: a large oval opening in line with the posterior end of the sella turcica that allows fibers of cranial nerve V (the trigeminal nerve) to pass to the chewing muscles of the lower jaw (mandible) 
Optic Canal: an opening that allows the optic nerve to pass to the eye
Superior Orbital Fissure: a slitlike opening through which the cranial nerves controlling eye movements (III, IV, and VI) pass
Sphenoid Sinuses: air cavities that riddle the central part of the sphenoid bone 
Ethmoid Bone: very irregularly shaped and lies anterior to the sphenoid ~ it forms the roof of the nasal cavity and part of the medial walls of the orbits
Crista Galli: a projection in which the outermost covering of the brain attaches
Cribriform Plates: allow nerve fibers carrying impulses from the olfactory (smell) receptors of the nose to reach the brain
Superior and Middle Nasal Conchae: form part of the lateral walls of the nasal cavity and increase the turbulence of air flowing through the nasal passage 

FACIAL BONES

Maxillary Bones: fuse to form the upper jaw
Alveolar Margin: where the maxillae carry the upper teeth
Palatine Processes: extensions of the maxillae that form the anterior part of  the hard palate of the mouth
Paranasal Sinuses: work to lighten the skull bones and probably act to amplify the sounds we make as we speak
Palatine Bones: form the posterior part of the hard palate ~ failure of these or the palatine processes to fuse medially results in cleft palate
Zygomatic Bones: commonly referred to as the cheekbones and form a good-sized portion of the lateral walls of the orbits or eye sockets 
Lacrimal Bones: fingernail-sized bones forming part of the medial walls of each orbit ~ each lacrimal bone has a groove that serves as a passageway for tears
Nasal Bones: small rectangular bones forming the bridge of the nose
Vomer Bone: the single bone in the median line of the nasal cavity which forms most of the nasal septum
Inferior Nasal Conchae: thin, curved bones projecting  from the lateral walls of the nasal cavity 
Mandible: the largest and strongest bone of the face that joins the temporal bones on each side of the face (forming the only freely movable joints in the skull) 
Hyoid Bone: the only bone of the body that does not articulate directly with any other bone, and is instead suspended in the mid-neck region where it is anchored by ligaments to the styloid processes of the temporal bones (is not technically part of the skull) 
Fetal Skull: the skull of a fetus or newborn infant that contains a small face, large total size in comparison to the rest of the body, and a lot of cartilage
Fontanels: the fibrous regions connecting the cranial bones

VERTEBRAL COLUMN (SPINE)

Vertebral Column/Spine: the axial support of the body that extends from the skull, which it supports, to the pelvis, where it transmits the weight of the body to the lower limbs
Vertebrae: the bones of the spine that are categorized under cervical, thoracic, and lumbar vertebrae 
Intervertebral Discs: pads of flexible fibrocartilage that cushion the vertebrae and absorb shocks while allowing the spine flexibility 
Primary Curvatures: the spinal curvatures in the thoracic and sacral regions
Secondary Curvatures: curvatures that develop when a baby begins to raise its head, and the lumbar curvatures begin to develop when the baby begins to walk
~~~ Body/Centrum: disclike, weight-bearing part of the vertebra facing anteriorly in the vertebral column
~~~ Vertebral Arch: arch formed from the joining of all posterior extensions from the vertebral body
~~~ Laminae/Pedicles: all posterior extensions
~~~ Vertebral Foramen: canal through which the spinal cord passes
~~~ Transverse Processes: two lateral projections from the vertebral arch
~~~ Spinous Process: single projection arising from the posterior aspect of the vertebral arch (actually the fused laminae) 
~~~ Superior and Inferior Articular Processes: paired projections lateral to the vertebral foramen, allowing a vertebra to form joints with adjacent vertebrae
Cervical Vertebrae: the seven vertebrae (C1-C7) that form the neck region of the spine
Atlas (C1): this joint allows you to nod "yes" 
Axis (C2): acts as a pivot for the rotation of the atlas (and skull) above (allows you to rotate your head from side to side indicating "no")
Dens/Odontoid Process: acts as the actual pivot point
Thoracic Vertebrae: the twelve vertebrae (T1-T12) that are larger than the cervical vertebrae (where the ribs are located) 
Lumbar Vertebrae: the five vertebrae (L1-L5) that have massive, blocklike bodies ~ their short, hatchet-shaped spinous processes make them look like a moose head from the lateral aspect
Sacrum: is formed by the fusion of five vertebrae ~ superiorly, it articulates with L5, and inferiorly, it connects with the coccyx
Alae: a winglike structure that articulates laterally with the hip bones, forming the sacroiliac joints
Median Sacral Crest: the fused spinous processes of the sacral vertebrae ~ this is flanked laterally by the posterior sacral foramina 
Sacral Canal: the vertebral canal that continues inside the sacrum
Sacral Hiatus: a large inferior opening that terminates the sacral canal
Coccyx: the fusion of three to five tiny, irregularly shaped vertebrae ~ it is the human "tailbone," a remnant of the tail that other vertebrate animals have
Bony Thorax: the sternum, ribs, and thoracic vertebrae
Thoracic Cage: a protective, cone-shaped cage of slender bones around the organs of the thoracic cavity (heart, lungs, and major blood vessels)
Sternum: a typical flat bone that is the result of the fusion of the three bones manubrium, body, and xiphoid process
~~~ Jugular Notch: the concave upper border of the manubrium that can be palpated easily
~~~ Sternal Angle: a transverse ridge that is formed by the manubrium and body meeting at a slight angle
~~~ Xiphisternal Joint: the point where the sternal body and xiphoid process fuse
Ribs: twelve pairs of bones that form the walls of the bony thorax ~ all the ribs articulate with the vertebral column posteriorly and then curve downward and toward the anterior body surface
True Ribs: the first seven pairs that attach directly to the sternum by costal cartilages
False Ribs: the next five pairs, either attached directly to the sternum or are not attached to the sternum at all
Floating Ribs: the last two pairs of false ribs that do not connect to the sternum at all

 

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Bone Markings

Projections That are Sites of Muscle and Ligament Attachment

--> Tuberosity: Large, rounded projection; may be roughened 
--> Crest: Narrow ridge of bone; usually prominent
--> Trochanter: Very large, blunt, irregularly shaped process (the only examples are on the femur)
--> Line: Narrow ridge of bone; less prominent than a crest
--> Tubercle: Small, rounded projection and process 
--> Epicondyle: Raised area on or above a condyle
--> Spine: Sharp, slender, often pointed projection
--> Process: Any bony prominence 

Projections That Help to Form Joints

--> Head: Bony expansion carried on a narrow neck
--> Facet: Smooth, nearly flat articular surface
--> Condyle: Rounded articular projection
--> Ramus: Armlike bar of bone

Depressions and Openings Allowing Blood Vessels and Nerves to Pass

--> Meatus: Canal-like passageway 
--> Sinus: Cavity within a bone, filled with air and lined with mucous membrane 
--> Fossa: Shallow, basinlike depression in a bone, often serving as an articular surface
--> Groove: Furrow
--> Fissure: Narrow, slitlike opening 
--> Foramen: Round or oval opening through bone

Structure of Long Bones

Diaphysis: the shaft that makes up most of the bones length and is composed of compact bone
Periosteum: the fibrous connective tissue membrane that covers and protects the diaphysis 
Perforating/Sharpey's Fibers: hundreds of connective fibers that secure the periosteum to the underlying bone
Epiphyses: the ends of the long bone ~ each epiphyses consists of a thin layer of compact bone enclosing an area filled with spongy bone
Articular Cartilage: (instead of periosteum) this covers the external surface of bones ~ because it is glassy hyaline cartilage, it provides a smooth, slippery surface that decreases friction at joint surfaces 
Epiphyseal Line: the line of bony tissue spanning the epiphysis that looks a bit different from the rest of the bone in that area ~ it is a remnant of the epiphyseal plate
Epiphyseal Plate: a flat plate of hyaline cartilage (seen in young, growing bone) that cause the lengthwise growth of a long bone ~ after puberty, the epiphyseal plate is replaced by bone, leaving only the epiphyseal lines to mark their previous location
Yellow Marrow or Medullary, Cavity: the cavity of the shaft that is used for storage of adipose (fat) tissue
Red Marrow: the substance found in bone cavities in infants ~ in adults, red marrow is confined to the cavities of spongy bone of flat bones and the epiphyses of some long bones
Bone Markings: the bumps, holes, and ridges that mark the outside of a bone and translate where muscles, tendons, and ligaments were attached/where blood vessels and nerves passed ~ the two categories of bone markings are projections/processes and depressions/cavities 
Osteocytes: mature bone cells
Lacunae: tiny cavities within the matrix where osteocytes can be found
Lamellae: the concentric circles that the lacunae form
Central (Haversian) Canals: what the lamellae form around ~ these canals run lengthwise through the bony matrix, carrying blood vessels and nerves to all areas of the bone
Osteon/Haversian System: each complex consisting of central canal and matrix rings
Canaliculi: tiny canals that radiate outward from the central canals to all lacunae ~ the canaliculi form a transportation system that connects all the bone cells to the nutrient supply through the hard bone matrix
Perforating (Volkmann's) Canals: canals that run into the compact bone at right angles to the shaft in order to create communication pathways from the outside bone to its interior (and the central canals)

The calcium salts deposited in the matrix give bone its hardness, whereas the organic parts (especially the collagen fibers) provide for bone's flexibility and great tensile strength.

 

Joints

Joints are the structures in the body that not only work to connect the bones to provide stability, but also to allow specific movements with the help of muscles. These structures come in three different categories known as the locked/immovable joints, passive movement/slightly movable joints, and freely movable joints (also known as synovial).

Immovable Joints

Suture- This is a type of fibrous joint that does allow movement (as they are only found on the skull, and the skull does require a minimal amount of flexibility), however so little that it is still categorized as immovable. The sutures are made up of Sharpey’s fibers which hod the bones of the skull together. When born, these suture sites are unfused, however in old age these cranial sutures may ossify completely.

Cartilage- These joints are made up of exactly what you’d expect… Cartilage! (the substance from which bone ossifies/Hyaline cartilage) While having more movement than fibrous joints like the one above, they still are quite limited (meaning that it can be found categorized in immovable or slightly movable). This joint is dispersed all throughout the body and is also responsible for laying out the growth regions of immature bones in children. In more specific terms, this cartilaginous joint is synchondrosis (meaning that it is solely joined by hyaline cartilage).

Slightly Movable Joints

Fibro-Cartilage- This type of joint seems to be exactly the same as the previous cartilage joint, however it is structurally different. These joints are associated with fibro-cartilage, a more spongy material. Because of this small difference, the joint is allowed more movement and is therefore categorized under “slightly movable.” In more specific terms, this cartilaginous joint is symphyses (meaning that although the end of the bones are lined with hyaline cartilage, they are joined by fibro-cartilage). These joints are found in between the vertebrae and pubic bones.

Freely Movable Joints

Plane- When trying to understand the plane joint, it helps to reference back to the planes associated with the directional terms. These planes are flat, and therefore this joint meets bones that are flat or nearly flat in relation to each other. This means that they can glide freely upon the plane that the joint connects them at. A better way to explain it is to imagine you are at a diner ordering coffee. Someone sets the coffee mug on the table and you slide it towards yourself. Now imagine that table being one bone and that mug being another bone. The joint is what creates the direction of the plane (aka the angle of the table surface) and allows the bones to glide (although movement is limited to the surrounding ligaments).

Hinge- The hinge joint (a much simpler joint) functions like a door hinge as it only allows the bones to move along one axis. Opening and closing the door is about the only thing you can do, and the same goes for the joint. The purpose of this is so that we can extend and flex our muscles. Examples of this joint would be the elbows and knees.

Saddle- Like the hinge joint, the saddle joint has a clear relation to real life objects. In this case, it is a bit more complex in comparison because it not only deals with a saddle, but also the person on it. Imagine two gears that fit together perfectly but allow enough space for movement along the sagittal and frontal planes. In relation to the saddle, just picture someone riding a horse. The saddle is the end of one bone and the crotch/legs are the end of the other bone. The best example of this joint is the base of the thumb.

Condyloid- This joint is a bit more complicated as it allows for more movement. The structure of it is is similar to a bowl. One bone has what seems to be a bowl on the end of it, and the other bone just happens to be able to fit it perfectly. It sounds a bit dumb with that explanation, but i mean, its accurate. Anyways, The best example of this joint would be the wrist as we can extend, flex, adduct, abduct, and allow circumduction. While it sounds a lot like the ball and socket joint, it is different as it does not allow axial rotation due to the muscles and ligaments surrounding (and it isn’t a perfect circle, it just deals with a spherical surface). In fact it has a closer resemblance to the saddle joint.

Pivot- These joints are responsible for allowing rotation throughout the body. By rotation, I mean how we can turn our head back and forth to communicate “no” to others, or how we can twist our elbows to allow supination and pronation. This also presents a good example of how we can have many joints at one part of the body (as the elbow has 3 of them alone).

Ball-And-Socket- This joint is one of the more versatile joints in the body as the range of motion and movement are wide. It is composed of one bone with a rounded end, and the other bone with a depression deep enough to fit it. The best examples of this joint are the shoulders and the hips (as they are the only examples available :-). And of course with this joint having the greatest freedom of them all, it can flex, extend, abduct, adduct, internally rotate as well as externally rotate.

Ellipsoid- Although there is already a misleading connection between the condyloid and ball-and-socket joints, this one adds to the mix anyways. Most of the time condyloid and ellipsoid are categorized under the same joint, however there are subtle differences. First of all, the ellipsoid deals with ellipsoidal surfaces while the condyloid deals with spherical. Carrying it over to the other side, the ball-and-socket joint is more like the ellipsoid than the condyloid. When it comes to the condyloid joint, it is actually an extension of the saddle joint because it adds more movement however contains a similar structure. When it comes to the ellipsoid joint, it is an alteration of the ball-and-socket joint as it has the same overall structure, just different shapes.

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