Phlox performs a pituitary gland transplant on Porthos
The pituitary gland is a segment of the brain which regulates hormones in the body and produces growth hormone. It is part of the endocrine system.
In 2152, when Phlox had to eradicate a pathogen in Porthos, it almost completely destroyed his pituitary gland. Since his Calrissian chameleon had a suitable one, he transplanted it from the chameleon to Porthos. (ENT: "A Night in Sickbay")