Intercostal muscles occur in Ribs.

The intercostal muscles are a group of muscles located between the ribs. They play a crucial role in the process of breathing by aiding in the expansion and contraction of the rib cage.

The intercostal muscles are divided into two main groups: external intercostal muscles and internal intercostal muscles.

External Intercostal Muscles:
- These muscles are located on the outer surface of the rib cage.
- They run obliquely downward and forward from one rib to the rib below it.
- The external intercostal muscles are responsible for elevating the rib cage during inhalation.
- When they contract, they lift the ribs upward and outward, expanding the chest cavity and allowing the lungs to fill with air.

Internal Intercostal Muscles:
- These muscles are located deep to the external intercostal muscles, between the ribs.
- They run obliquely downward and backward from one rib to the rib above it.
- The internal intercostal muscles are responsible for depressing the rib cage during forced exhalation.
- When they contract, they pull the ribs downward and inward, decreasing the size of the chest cavity and helping to expel air from the lungs.

Together, the external and internal intercostal muscles work in coordination to facilitate the process of breathing. During inhalation, the external intercostal muscles contract, lifting the rib cage and expanding the chest cavity, while the internal intercostal muscles relax. During exhalation, the internal intercostal muscles contract, depressing the rib cage and reducing the size of the chest cavity, while the external intercostal muscles relax.

In conclusion, intercostal muscles occur in the ribs and are essential for the mechanism of breathing by aiding in the expansion and contraction of the rib cage.