Three important spaces of the shoulder that are bordered by the triceps include. quadrangular space; triangular space; triangular interval.
Boundaries and contents of the spaces
1. Quadrangular space
They are paired, quadrangular shaped spaces seen near the axillary region. The space is mainly formed by the triceps muscle and the humerus. Hence it is also called foramen humero-tricipitale.
Boundaries of the quadrangular space
As the name suggests, the quadrangular spaces has four sides or boundaries. The table below lists out the boundaries of the space. Use the image to understand better.
| Above | Subscapularis muscle in front Capsule of shoulder joint Teres minor muscle behind |
| Below | Teres major |
| Medial | Long head of triceps brachii |
| Lateral | Surgical neck of the humerus |
2. Upper triangular space
The upper triangular space is the space seen between the two teres muscles.
Boundaries of the upper triangular space
As the name suggests, the upper triangular space has two sides, a base and an apex. The table below lists out the boundaries of the space. Use the image to understand better.
| Above | Teres minor |
| Below | Teres major |
| Apex | Meeting point of teres minor and major muscles. This can be pointed to on the lateral border of the scapula. |
| Base (lateral boundary) | Long head of triceps brachii |
3. Lower triangular space
It lies below the quadrangular space. It is formed due to the teres major muscle cutting across the long head of triceps brachii to reach its insertion.
Boundaries of the lower triangular space
As the name suggests, the lower triangular space has two sides, a base and an apex. The table below lists out the boundaries of the space. Use the image to understand better.
| Above (base) | Teres major |
| Medial | Long head of triceps |
| Lateral | Shaft of the humerus |
| Apex | Meeting point of the medial and lateral boundaries |
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