Educational resource developed at the Veterinary Anatomy Section, ICAR-Indian Veterinary Research Institute (ICAR-IVRI), Bareilly.
📝 STANDARD DESCRIPTION:
By Shape:
• Fusiform — spindle-shaped (biceps).
• Triangular — fan-shaped (pectorals).
• Quadrilateral — square (rhomboideus).
• Strap-like — long, parallel (sartorius, sternohyoideus).
• Pennate — feather-like:
– Unipennate (one side).
– Bipennate (both sides).
– Multipennate (many).
• Circular (sphincter) — ring (orbicularis oculi).
By Action:
• Flexor / Extensor — flex / extend joint.
• Adductor / Abductor.
• Sphincter / Dilator.
• Levator / Depressor.
• Rotator (medial / lateral).
• Tensor.
Muscle Naming (Etymology): Latin/Greek roots indicating shape, location, action, attachments, or number of heads.
📚 DETAILED DESCRIPTION:
MUSCLE CLASSIFICATION & ETYMOLOGY
I. BY SHAPE:
• Fusiform — central belly tapering to tendons (biceps brachii).
• Strap-shaped — flat, parallel fibres (sternohyoideus, sartorius).
• Triangular — broad origin, narrow insertion (deltoideus).
• Quadrilateral — rectangular (rhomboideus).
• Pennate (feather-shape):
– Unipennate (semimembranosus).
– Bipennate (rectus femoris).
– Multipennate (deltoideus in humans).
• Circular / Sphincter — orbicularis oris/oculi, anal sphincter.
• Convergent — broad origin to narrow insertion.
II. BY NUMBER OF HEADS:
• Bi-ceps (2 heads) — Biceps brachii, biceps femoris.
• Tri-ceps (3) — Triceps brachii, triceps surae.
• Quadri-ceps (4) — Quadriceps femoris.
III. BY ACTION:
• Flexor / Extensor — joint angle changes.
• Abductor / Adductor.
• Levator / Depressor — raise / lower (eyelids, ribs).
• Sphincter (constrictor) / Dilator.
• Rotator (medial / lateral).
• Tensor — tightens fascia (tensor fasciae latae).
• Pronator / Supinator.
• Antagonist / Agonist / Synergist / Fixator.
IV. BY LOCATION:
• Brachii (arm), femoris (thigh), oculi (eye), oris (mouth), thoracis (chest).
• Superficialis / profundus (superficial / deep).
• Externus / internus.
• Anterior / posterior.
V. BY ATTACHMENTS:
• Sterno-cleido-mastoid (sternum + clavicle + mastoid).
• Sterno-cephalicus.
• Coraco-brachialis.
EPONYMOUS MUSCLES: A few are named after persons or features:
• 'Common digital extensor', 'Lateral digital extensor', 'Cutaneous trunci' (for fly-swatting reflex).
RECIPROCAL APPARATUS (in horse hindlimb): Mechanical linkage of peroneus tertius (cranial) + superficial digital flexor (caudal) so that flexion of the hock automatically flexes the stifle.
// COMPARATIVE ANALYSIS
| HORSE |
Stay apparatus = cooperative ligament+muscle system that lets the horse sleep standing (key UG concept). |
| DOG |
Brachycephalic dogs have hypertrophied pterygoid muscles for bite strength. |
| BIRD |
Pectoralis major (downstroke) and supracoracoideus (upstroke) are the chief flight muscles. |
CLINICAL RELEVANCE
Atrophy = wasting of muscle (denervation, disuse). Hypertrophy = increase in fibre size (training, anabolic agents). Contracture = permanent shortening (post-trauma fibrosis). Myositis = inflammation.
