The phrase "medial part" refers to the middle or central section of something, often used to describe a body part or an object that is situated closer to the center. Full definition
The diaphragm has multiple origins from the inner surfaces of the seventh through twelfth ribs, medial parts of the L1 to L3 vertebral bodies, the anterior longitudinal ligament, posterior surface of the xiphoid process, and the arcuate ligament, connecting to the aorta, psoas, and QL to insert in the central tendon. (breakingmuscle.com)
Neurons from the distal part of the antenna innervate the medial part of the macroglomerulus, whereas neurons from the more proximal part of the antenna innervate the more lateral portion. (sciencedaily.com)