Anatomy of the Human Skeleton and Dentition

The Skeleton: Post-Cranial and Cranial Anatomy

Introduction to Skeletal Anatomy

• Understanding evolutionary history, diversity, and adaptations of primates involves studying anatomical evidence.

• For paleontologists, anatomy, particularly of fossils, is highly relevant.

The Post-Cranial Skeleton

• This part of the skeleton includes everything except the skull.

• It is part of the axial skeleton, which includes the vertebral column, ribs, and sternum.

The Vertebral Column (Spine)

• The vertebral column is a stack of individual bones called vertebrae.

• Vertebrae are sorted into different regions:

  • Cervical Vertebrae:

    • Located in the neck, just below the head.

    • There are $7$ cervical vertebrae.

  • Thoracic Vertebrae:

    • Located in the chest area.

    • There are $12$ thoracic vertebrae.

    • Each thoracic vertebra articulates with a pair of ribs.

  • Lumbar Vertebrae:

    • Located in the lower back, forming an inward curvature.

    • There are $5$ large lumbar vertebrae.

    • Vertebrae generally get larger as they descend the column because they support increasing body weight.

  • Sacrum:

    • A triangular plate of bone below the lumbar vertebrae.

    • Represents the fusion of $5$ vertebrae, treated as a single structure.

  • Coccygeal Vertebrae:

    • Located at the very bottom, a small nub on the sacrum.

    • Consists of $3$ to $4$ variable, tiny bones.

    • These are the vestigial remains of a tail, often called the tailbone.

The Thoracic Cage (Chest)

• Composed of:

  • $12$ thoracic vertebrae.

  • $12$ pairs of ribs (on either side) which are long, bony struts.

  • The sternum (breastbone): A bony plate in the front-center of the chest. It can be divided into three parts but is often considered one bone. Ribs attach to the sternum indirectly via cartilaginous extensions.

• Together, these structures form a protective cage, the thoracic cage.

The Upper Limb

• Scapula (Shoulder Blade):

  • A triangular, flat plate of bone primarily visible from the back.

  • Contains a socket that forms part of the shoulder joint (a ball-and-socket joint).

• Clavicle (Collarbone):

  • A longitudinal strut of bone running between the sternum and the scapula, above the shoulder joint.

  • Serves to push the shoulder joint laterally and keep it in place.

  • A shoulder separation occurs when ligaments holding the scapula and clavicle together are torn.

• Shoulder Girdle: Collective term for the scapula and clavicle.

• Humerus:

  • The single bone of the arm (the segment between the shoulder and elbow joints).

  • Its rounded ball-like end articulates with the scapula to form the shoulder joint.

• Forearm: The segment of the upper limb between the elbow and wrist joints.

  • Radius: The bone on the thumb side (lateral side) of the forearm.

  • Ulna: The bone on the pinky finger side (medial side) of the forearm.

• Wrist and Hand Bones:

  • Carpals: $8$ small, irregularly shaped bones forming the wrist joint, articulating principally with the radius.

  • Metacarpals: $5$ longish bones forming the hard part of the back and palm of the hand, beyond the carpals.

  • Phalanges:

    • The bones of the fingers.

    • Digits $2$ through $5$ (index, middle, ring, pinky) each have $3$ phalanges.

    • Digit $1$ (thumb) has $2$ phalanges.

    • Total of $14$ phalanges in one hand.

The Lower Limb

• Os Coxa (Pelvic Bone/Hip Bone):

  • Large, irregularly shaped bone, with one on either side fused together in adults.

  • Each os coxa represents the fusion of three separate bones:

    • Ilium: The curved, paddle-like upper portion.

    • Ischium: The lower, projecting portion on which one sits.

    • Pubis: The bar of bone that curls around to the front.

  • Contains a large socket, the acetabulum, which is the socket for the hip joint (another ball-and-socket joint).

• Femur:

  • The single bone of the thigh (the segment between the hip and knee joints).

  • It is the largest bone in the body, with a rounded ball-like end articulating with the os coxa.

  • The leg (from hip to toes) is referred to as the lower limb.

• Patella (Kneecap):

  • A peculiar small bone at the knee joint, formed within a muscular tendon.

• Leg: The segment of the lower limb between the knee and ankle joints.

  • Tibia (Shinbone):

    • The larger of the two leg bones, primarily responsible for weight-bearing.

    • Often easily felt as a raw, flat bony surface under the skin.

    • Example: A child's tibialis anterior muscle might hypertrophy from dorsiflexion (toes pointing up in stirrups) to cover the shin bone.

  • Fibula: The second leg bone, not explicitly named in the transcript, but implied as the other bone in the leg.

• Ankle and Foot Bones:

  • Tarsals: $7$ (not $8$ like carpals) bones that form the ankle joint and part of the foot.

  • Metatarsals: $5$ straight bones that make up the main part of the top of the foot, beyond the tarsals.

  • Phalanges: The bones of the toes, similar in structure and naming to finger phalanges.

The Skull

• The bones of the head are divided into two main categories:

  • Mandible: The lower jaw bone, which is mobile and holds the lower teeth.

  • Cranium: All other bones of the skull, excluding the mandible and its teeth.

    • Skull = Cranium + Mandible.

• Most cranial bones are immobile and tightly fixed by fibrous joints called sutures.

• The temporomandibular joint is where the mandible articulates with the cranium.

Bones of the Cranium

• Frontal Bone: A single bone forming the forehead and extending back over the crown of the head. It also forms part of the roof and brow ridge of the orbit (eye cavity).

• Parietal Bones: A pair of bones on the sides of the top of the head, one on either side, behind the frontal bone.

• Occipital Bone: A single bone at the back and bottom of the skull, extending from both right and left sides.

• Temporal Bones: Very irregular bones on either side of the cranium, making up the vertical side part (not the temple of the head). They contain important features like the external auditory meatus (ear canal).

• Zygomatic Bone (Cheekbone):

  • Forms part of the lateral wall of the orbit.

  • Has a bony extension that extends back towards the ear.

  • Articulates with a process from the temporal bone.

• Maxilla:

  • Irregularly shaped bone on either side, more towards the midline.

  • Contributes to the inferior margin of the orbit and the sides of the nasal cavity.

  • Contains sockets for the upper teeth.

• Sphenoid Bone: A complicated, single, very irregular bone projecting out to the side and seen on the bottom of the skull (mentioned but not detailed further).

Dentition (Teeth)

• Teeth are housed in the maxilla (upper teeth) and the mandible (lower teeth).

• The human dentition is heterodont, meaning there are different types of teeth with different shapes for different functions.

  • This contrasts with homodont vertebrates, which have only one type of tooth (e.g., conical).

• The dental arrangement can be divided into four quadrants: Upper right, upper left, lower right, lower left.

  • Symmetry typically exists between right and left sides in mammals, and in primates, often between upper and lower sets as well.

Types of Teeth and Their Functions

• Incisors (I):

  • Located at the very front of the mouth.

  • Broad, flat, spatula-shaped, with a long cutting edge.

  • Used for incising (cutting) bite-sized chunks from food that isn't too hard.

    • Example: Not used to bite into a hard nut due to risk of breakage and a nervous system feedback loop preventing excessive force.

• Canines (C):

  • Next to the incisors, moving distally (towards the back) along the tooth row.

  • In humans, they are relatively blunt and small.

  • In many mammals (e.g., dogs, cats), they are large and pointy, used for display and killing prey.

• Premolars (P):

  • Located behind the canines.

  • Known as