The hierarchy is designed to allow a MEDLINE/PubMed search of a broader term to include the narrower terms in all branches automatically. This is known as explosion.

For example, a search of "ear" in PubMed would automatically explode to include records indexed with Ear, External; Ear, Middle; and Ear, Inner, as well as all narrower terms under each of these.

Anatomy
	Sense Organs
		Ear
			Ear, External + Ear, Middle + Ear, Inner +