In anatomy, the intercostal space, often abbreviated ICS, is the space between any two ribs. This term comes from the Latin roots "inter," meaning between, and "costa," meaning a rib. Humans have 12 ribs on either side, meaning that there are 11 intercostal spaces on each side of the chest, numbered from first to eleventh according to the number of the rib above the space. The intercostal space houses a number of different anatomical features.
Each of these spaces contains several layers of intercostal muscles, each of which has a different specific function. The diaphragm combine with the intercostal combine to play a key role in breathing. These muscles move the chest, increasing and decreasing the size of the chest cavity. Expanding and contracting the chest draws air into the lungs and forces it out again. The intercostal muscles are surrounded by two membranes called the external and internal intercostal membranes.
Each layer of intercostal muscle plays a different role in the process of breathing. The external intercostal muscles lift the ribs, expanding the chest cavity for inhalation. The internal intercostal muscles lower the ribs as part of exhalation. The last layer, the innermost intercostal muscles, contains other muscles which also play a role in inhalation, as well as one, the subcostalis muscle, the function of which is unclear. Intercostal nerves and arteries connect to the intercostal muscles, supplying blood and controlling their actions.
In addition to layers of muscle, the intercostal space contains bundles of nerves, veins and arteries. These run together in a grouping called a neurovascular bundle. The neurovascular bundle runs between the internal and innermost intercostal muscles in the upper part of the intercostal space, close to the rib. The vein occupies the uppermost part of the neurovascular bundle, with the artery beneath it and the nerve at the bottom. As a result of this position, surgical insertions into or through the intercostal space usually pass between the ribs at a low angle to avoid touching the neurovascular bundle.
In animals other than humans, the anatomy of the intercostal space varies. Some, such as dogs, cats, rabbits and monkeys have an anatomy very similar to humans. In others, however, the muscles of the intercostal space are arranged very differently, although the function appears to be the same. This is the case in goats and sheep, which lack the layered intercostal muscles found in humans.