SKELETAL

MODULE 6: MUSCULOSKELETAL SYSTEM


The skeleton is an essential part of the body of every vertebrate animal. It has several functions:

  1. Serves as a support or framework for the softer parts of the body.

  2. Provides a firm surface for the attachment of muscles used in movement and locomotion.

  3. Supplies calcium to the blood.

  4. Important site in the formation of blood cells.

There are two types of skeletons: the exoskeleton and the endoskeleton.


Exoskeleton: Consists of hardened bony or horny structures that develop in the skin and provide external protection. These structures are poorly developed in frogs and toads.


Endoskeleton: The skeleton of the frog/toad is almost entirely endoskeletal, consisting mostly of bone and cartilage.

Skeletal muscles and the skeleton are often considered together as a functional unit that permits body movements and locomotion. Muscles are attached to the bones by tendons. A broad, flat, and sheet-like tendon is called an aponeurosis. Skeletal muscles often have two points of attachment, with the part in between sometimes referred to as the belly of the muscle. Locating the attachments can facilitate identification of a muscle. Other clues include the shape and relative size of the muscles, the direction of the muscle fibers, and the action of the muscles. 


These factors are also the bases for naming muscles.


A. Gross Anatomy of the Toad Skeleton


Axial Endoskeleton: Composed of all endoskeletal structures located at the center or axis of the body.

  • Skull

  • Visceral skeleton

  • Vertebral column and ribs (lacks true ribs)

  • Sternum

  • Appendicular Endoskeleton: Consists of all endoskeletal structures located laterally or in the region of the extremities or appendages.


Girdles: Pectoral girdle, Pelvic girdle

Bones of the limbs: Anterior limbs (forelimbs), Posterior limbs (hindlimbs)

Skull

Dorsal View:

Nasal bones: Flat, triangular-shaped bones behind the nares.

Frontoparietals: Long, slender, flat bones posterior to the nasal bones.

Foramen magnum: Large opening at the posterior end of the skull for the spinal cord.

Exoccipital: Bone surrounding the posterior part of the brain, with occipital condyles projecting from it.

Premaxillae: Paired, short, teeth-bearing bones forming the anterior edge of the upper jaw.

Maxillae: Paired, curved, teeth-bearing bones forming the margin of the jaw.

Quadratojugals: Short bones making up the posterior part of the upper jaw, without teeth.

Squamosals: Hammer-shaped bones that brace the cranium and jaws.

Orbits: Large openings lateral to the frontoparietals for the eyeballs.

Ventral View:

Vomers: Wing-shaped bones in the roof of the mouth, bearing vomerine teeth.

Palatines: Slender, rod-like bones on the lower side of the cranium.

Parasphenoid: Dagger-shaped single bone forming the floor of the cranium.

Visceral Skeleton

Derived from the gill arches, consisting of:

Upper and lower jaws

Hyoid apparatus: Supports muscles for respiratory movements of the mouth.

Cartilages: Support the ventral wall of the larynx.

Vertebral Column

Specialized structure due to the shortening of the trunk.

Ten vertebrae:

Atlas: First vertebra, articulates with the skull.

Urostyle: Fused caudal vertebrae forming a conspicuous dorsal keel.

Pectoral Girdle

Supports the forelimbs and is attached to the sternum.

Pelvic Girdle

Supports the hindlimbs, each half called an innominate bone.

Bones of the Limbs

Forelimbs:

Humerus: Bone of the upper arm.

Radioulna: Fusion of radius and ulna, with an olecranon process for muscle attachment.

Carpals: Bones of the wrist.

Metacarpals and Phalanges: Bones of the hand and fingers.

Hindlimbs:

Femur: Bone of the thigh.

Tibiofibula: Fusion of tibia and fibula.

Tarsals: Anklebones, with elongated proximal tarsals (astragalus and calcaneus).

Metatarsals and Phalanges: Bones of the sole and toes.

B. Skeletal Muscles of the Toad

Skinning the Specimen:

Rinse the preserved toad and make careful incisions to remove the skin.

Dissection Hints:

Use blunt instruments to separate muscle bundles.

Avoid cutting muscles unless necessary, and if so, cut through the belly of the muscle.

Identification of Muscles:

Ventral Muscles of the Head and Trunk:

Mylohyoid: Transversely oriented fibers lateral to the median raphe on the lower jaw.

Sternoradialis: Thick median pair of muscles, partly covered by the mylohyoid.

Cutaneous pectoralis: Most ventral, thin, pocket-like superficial muscle.

Pectoralis: Fan-shaped muscles posterior to the sternoradialis, divisible into three parts.

Rectus abdominis: Longitudinal muscle lateral to the linea alba.

External oblique: Sheet-like muscle lateral to the rectus abdominis, with oblique fibers.

Ventral Muscles of the Thigh and Shank:

Sartorius: Flat, strap-like muscle on the ventral side of the thigh.

Adductor longus: Slender muscle beneath the sartorius.

Adductor magnus: Thick muscle near the groin area, posterior to the adductor longus.

Gracilis major: Large muscle with oblique fibers posterior to the adductor magnus.

Gastrocnemius: Large muscle at the back of the shank.

Tibialis posticus: Long, narrow muscle anterior to the gastrocnemius.

Tibialis anticus: Most anterior muscle of the shank, splitting distally.

Dorsal Muscles of the Head and Trunk:

Longissimus dorsi: Mass of muscle alongside the mid-dorsal line, tapering posteriorly.

Iliolumbaris: Muscle lateral to the longissimus dorsi.

Coccygeo-iliacus: Pair of large V-shaped muscles between the ilia of the pelvic girdle.

Dorsal Muscles of the Thigh and Shank:

Triceps femoris: Largest, three-headed muscle on the lateral surface of the thigh.

Gluteus: Small, thick muscle between the heads of the triceps femoris.

Biceps femoris (Iliofibularis): Slender muscle posterior to the triceps femoris.

Semimembranosus: Large muscle next to the biceps femoris.

Gastrocnemius: As previously described.

Peroneus: Muscle of the shank anterior to the gastrocnemius.

Tibialis anticus: As previously described.