Overview
It is convex cranially. Its periphery is muscular and its centre is a tendon (centrum tendineum). It has three openings: the aortic hiatus, the oesophageal hiatus and the caval foramen.
In the horse the line of attachment is oblique: ventrally it attaches to the xiphoid process and from there it rises caudodorsally along the costal cartilages and the costal arch to the last ribs. The dome projects far cranially, so the intrathoracic part of the abdomen is large: the liver, the stomach and much of the spleen rest on its abdominal surface, under the ribs. On its thoracic surface, the lungs fill the costodiaphragmatic recesses and the accessory lobe fills the mediastinal recess.
Origin
- Lumbar part: the right and left crura, on the bodies of the cranial lumbar vertebrae.
- Costal part: medial surface of the caudal ribs and their cartilages, along the costal arch.
- Sternal part: xiphoid process.
Insertion
Central tendon.
Action
When it contracts it flattens, enlarges the thoracic volume and produces inspiration.
Innervation
Phrenic nerve (n. phrenicus), formed by ventral branches of the caudal cervical nerves.
Clinical relevance
- Diaphragmatic hernia: congenital or acquired through trauma, falls, foaling or strenuous exertion. Loops of small intestine, or less often colon, pass into the thorax and may become strangulated; the main sign is colic, sometimes with respiratory difficulty. It is diagnosed by ultrasound or radiography and repaired surgically.
- Synchronous diaphragmatic flutter ("thumps"): the phrenic nerve depolarises with each heartbeat because it runs over the heart, and the flank twitches in time with the heart. It appears with calcium and other electrolyte disturbances, typically in exhausted endurance horses, and in hypocalcaemia of the lactating mare.