📝 STANDARD DESCRIPTION:
Type: The MAIN muscle of breathing (chief inspiratory muscle).
Shape: Dome-shaped sheet between thorax and abdomen.
Parts:
- Muscular periphery: Costal, Sternal, Lumbar (Crura) parts.
- Tendinous Centre (Centrum tendineum): Y-shaped central tendon.
Three Holes (very important):
1. Aortic hiatus (most dorsal, in lumbar part) — passes Aorta, Azygos vein, Thoracic duct.
2. Oesophageal hiatus (middle) — passes Oesophagus + Vagus trunks.
3. Caval foramen (most ventral, in tendinous centre) — passes Caudal Vena Cava.
Function: Contracts → flattens → enlarges thorax → INSPIRATION.
Nerve: Phrenic nerve (C5-C7).
📚 DETAILED DESCRIPTION:
THE BIG BREATHING MUSCLE — DIAPHRAGM
The diaphragm is a thin, dome-shaped muscular sheet that completely separates the thoracic cavity (chest) from the abdominal cavity (belly). It is the most important muscle of inspiration — about 75% of the air pulled into the lungs comes from its action.
STRUCTURE:
• Muscular Rim (peripheral):
– Costal part: Arises from inner surfaces of the last few ribs and costal cartilages.
– Sternal part: Arises from xiphoid cartilage of sternum.
– Lumbar part (Crura): Arises from ventral surfaces of first few lumbar vertebrae as two pillar-like 'crura' (right crus is larger and longer than left).
• Tendinous Centre: A Y-shaped sheet of tendon in the middle where all the muscle fibres converge. Cannot contract — acts as the 'roof' of the dome.
THE THREE HOLES (HIATUSES) — MUST REMEMBER:
From dorsal to ventral:
1. Aortic Hiatus — most dorsal, in the lumbar part between the two crura. Passes the AORTA, AZYGOS vein, and THORACIC DUCT.
2. Oesophageal Hiatus — in the right crus, middle level. Passes the OESOPHAGUS and the two VAGAL TRUNKS (dorsal and ventral).
3. Caval Foramen — most ventral, in the tendinous centre itself. Passes only the CAUDAL VENA CAVA (which is fixed to the tendon so it cannot collapse).
HOW IT WORKS: The relaxed diaphragm bulges forward into the thorax (dome). When it contracts, the dome flattens and is pulled backward (toward the abdomen). This increases the front-to-back length of the chest → air is sucked into the lungs.
NERVE SUPPLY: Phrenic nerve (from cervical spinal nerves C5, C6, C7). Coming from the neck, the phrenic runs all the way down through the chest to reach the diaphragm.
TIP: Highest point of the dome reaches forward as far as the 6th-7th rib in the standing animal.
// COMPARATIVE ANALYSIS
| HORSE |
Very strong; right crus is huge and partly attached to last ribs (helps in galloping rhythm — feet hit ground in time with diaphragm push). |
| DOG |
Smaller animal but diaphragm does ~90% of breathing work at rest. |
| PIG |
Right crus very large; oesophageal hiatus relatively narrow. |
| FOWL |
NO TRUE DIAPHRAGM. Replaced by thin pulmonary aponeurosis. Birds breathe using rib movement and air sacs only. |
CLINICAL RELEVANCE
Diaphragmatic Hernia: A tear in the diaphragm (e.g. after road accident in dog, or after calving in cattle) lets abdominal organs slip into the chest — causes severe breathing difficulty.
Phrenic nerve injury: Paralyses one half of the diaphragm — that side moves up instead of down during breathing (paradoxical movement). 'Hiccups' = repeated jerky contractions of the diaphragm.
