Final answer:
The term "Inter+costal" refers to the spaces between the ribs, and the intercostal muscles consist of three sets (external, internal, and innermost) that span these spaces, assisting in breathing by changing the rib cage dimensions.
Step-by-step explanation:
"Inter+costal" means between the ribs. There are three sets of intercostal muscles: external, internal, and innermost intercostals. The external intercostals are located on the sides of the body, and they aid in inspiration by raising the rib cage. Conversely, the internal intercostals are situated just below the external ones and are used for expiration as they draw the ribs together, constricting the rib cage.
The innermost intercostals lie deep to both the internal and external intercostals, acting as synergists for the action of the internal intercostals. All three sets span each of the intercostal spaces, which are the gaps between the ribs, and play a pivotal role in changing the rib cage dimensions to facilitate breathing.