In many medical terms, what joins a root word to a suffix?

In many medical terms, a combining vowel joins a root word to a suffix.

A combining vowel (usually O or I and less frequently U) is used between two elements of a medical term to make the term easier to pronounce. The other vowels may also be used, but less often.

Example:

hepatmalacia (hepat/malacia) - - hepatomalacia (hepat/o/malcia)