Toad Skeletal System: Axial Components

• The toad skeletal system contains axial and appendicular components.

Axial Skeleton

• The skull is flattened dorsoventrally and degenerate.
• Many bones found in most amphibians are lost or heavily modified in toads.

Skull Components

• Exoccipitals: Two flat bones bordering the foramen magnum (the large opening at the posterior of the skull for the spinal cord).
  • They articulate with the atlas, the toad's first and only cervical vertebra.
• Fronto-parietal: A pair of elongated bones anterior to the exoccipitals.
  • Formed by the fusion of the frontal and parietal bones, they form the dorsal covering of the skull.
• Maxilla: An elongated bone located posterior-lateral to the pre-maxillae (the pair of bones forming the upper jaw).
  • Maxilla form the greater part of the upper jaw and bear teeth in frogs, but not in toads.
• The visceral apparatus is composed of the lower jaw and the hyoid cartilage.
• It functions to support the anterior portion of the digestive tract.

Visceral Apparatus Components

• Lower jaw (mandible): A component of the visceral apparatus, made of right and left halves.
• Hyoid cartilage: Another component of the visceral apparatus.
  • A thin cartilaginous plate supporting the floor of the mouth.
• The vertebral column of toads or frogs is made up of 10 vertebrae.

Vertebral Column Components

• Atlas: The first and only cervical vertebra in toads and frogs, lacking processes.
  • It has a pair of depressions on its anterior surface for articulation with the exoccipital condyles of the skull.
• Typical vertebrae: Consisting of the 2nd to the 8th vertebra.
• Sacral vertebra: The 9th vertebra, characterized by flat, expanded ends of the transverse processes.
  • The ilia of the pelvic girdles articulate with the processes and the posterior ends of the centrum are made of two convexities which articulate with the two concavities of the tenth vertebra.
• Urostyle: This is the 10th vertebra.
  • It’s an elongated bone located between the anterior portions of the pelvic girdle.
  • Corresponds to the caudal vertebrae in other vertebrates and has a pair of small openings on the sides near its anterior portion for the passage of the 10th pair of spinal nerves.

Appendicular Skeleton

• The pectoral girdle is a series of bones that supports the anterior appendages or forelimbs.
• It isn't attached to the ventral column except through muscles and ligaments.
• Each girdle is composed of left and right halves.

Pectoral Girdle Components

• Scapula: A short, flat bone below the suprascapula (a flattened structure or cartilage located at the dorsal part of the girdle).
• Clavicle: A slender bone found on the anterior median ventral part of the girdle.
• Sternum: Usually attached to the pectoral girdle at the median posteroventral side.

Forelimb Components

• It's composed of six separate bones.
• Humerus: The single long bone of the upper arm with a rounded head at its proximal end.
  • The head fits into the glenoid fossa (a depression formed at the junction between the scapula, clavicle, and coracoid) of the pectoral girdle.
  • A prominent ridge, the deltoid ridge, is on its anterior surface for muscle attachment.
• Radio-ulna: Forearm bone formed by the fusion of the radius and ulna, which are separate in other vertebrates.
  • The radius is inner, the ulna is outer.
• Carpals (Proximal/Distal): Six short bones of the wrist attached to the distal end of the radio-ulna and arranged in two rows.
• Metacarpals: Bones of the palms which form the basal segments of the digits.
• Phalanges: Short bones of the fingers beyond the metacarpals (singular: phalanx).

Pelvic Girdle

• Articulates to the transverse process of the sacral vertebra, supporting the hindlimbs.
• It has right and left halves, each known as the innominate bone.

Pelvic Girdle Components

• Ilium (pl. ilia): A long bone articulating with the transverse process of the sacral vertebra.
• It forms the anterior border of the acetabulum (a depression that receives the head of the femur).
• Ischium (pl. ischia): A posterior flat bone forming the posterior border of the acetabulum.
  • The two ischia fuse at the mid-ventral portion via the ischiac symphysis.
• Pubis (pl. pubes): Triangular bones on the ventral side of the pelvic girdle.
  • They forms the ventral border of the acetabulum.
  • The two pubes are united at the mid-ventral portion of the girdle by the pubic symphysis.

Hindlimb Components

• Composed of six separate bones.
• Femur: The thigh bone with a rounded head at its proximal end, fitting into the acetabulum.
• Tibio-fibula: The shank bone formed by the union of the tibia and fibula of other vertebrates.
  • The tibia is inner, the fibula is outer.
• Metatarsals: The bones of the sole which are the basal segments of the toes, beyond the tarsals.
  • There are five metatarsals.
• Phalanges: The short bones constituting the toes.
  • The digits and the phalanges in each of the digits determine their number.

Overview of the toad's skeletal system, focusing on the axial components and skull structure. Details the exoccipitals, fronto-parietal bones, maxilla, and visceral apparatus. Highlights modifications and losses of bones compared to other amphibians.