Accessory breathing muscles:

• Serratus Dorsalis Cranialis — Origin: thoracolumbar fascia; Insertion: caudal border of cranial ribs. INSPIRATORY (pulls ribs forward).

• Serratus Dorsalis Caudalis — Origin: thoracolumbar fascia; Insertion: cranial border of caudal ribs. EXPIRATORY (pulls ribs backward).

• Transversus Thoracis — Lies on inside of sternum; depresses ribs in expiration.

• Scalenus — Cervical to first ribs; fixes first ribs during forced inspiration.

• Rectus Thoracis — Small muscle from 1st rib to costochondral junctions of cranial ribs; assists inspiration.

HELPER MUSCLES OF BREATHING

When the animal breathes hard (running, fever, dyspnoea), the diaphragm and intercostals get help from accessory respiratory muscles. These act on the rib cage from outside.

SERRATUS DORSALIS CRANIALIS (Inspiratory):
Triangular muscle in the dorsal thoracic wall. Origin from the thoracolumbar fascia near the withers; fibres run downward and backward to insert on the cranial ribs. Pulls the ribs forward and outward — helps inspiration.

SERRATUS DORSALIS CAUDALIS (Expiratory):
Lies more caudally; fibres run downward and forward (opposite direction). Pulls the caudal ribs backward — helps forced expiration. Easy to remember: cranialis pulls forward = inspiration; caudalis pulls backward = expiration.

TRANSVERSUS THORACIS:
Thin sheet on the deep (inside) face of the sternum and costal cartilages. Pulls ribs ventrally → expiration.

SCALENUS:
Runs from transverse processes of last cervical vertebrae to the 1st (and sometimes 2nd-4th) ribs. Fixes the first rib so the intercostals have a stable anchor to pull against during deep breathing.

RECTUS THORACIS:
Small strap-like muscle on the ventral chest wall, from 1st rib to costal cartilages of cranial ribs. Minor inspiratory role.

CLINICAL TIP: A sick animal in respiratory distress shows visible 'pumping' of the flanks and 'flaring' of the nostrils — this is recruitment of accessory muscles. It is a sign of severe breathing trouble.