"The power infused into you will make some changes. These will not only be physical changes. Mental changes must be expected."
"Oh," Forrester said. "Mental changes."
"Correct. Physically, you see, you will become what no mortal can ever quite be: a perfectly functioning biological engine. Every sinew, nerve and muscle, every organ and gland, every tissue in your body will be in perfect harmonic balance with every other. Metabolically speaking, your catabolism and anabolism will be in such perfect balance that aging will not be possible."
Forrester thought that over. "I'll be immortal," he said.
"In that sense of the word," Vulcan said, "you will. You will be, as a matter of fact, quite a good deal tougher, stronger and harder than any animal now existing on the face of the Earth. I must except, of course, a few of the really big ones, like the elephant and the killer whale."
"Oh," Forrester said. "Sure."
"But make no mistake. You can still be killed. A bullet through the heart will not do the job; it will merely incapacitate you for a few hours. But if you were to have your head blown off by a grenade, you would be quite dead. Remember that."
"I don't see how I could forget it."
"You will heal with incredible rapidity, but there are limitations. Anything that pushes the balance too far will be fatal. You can lose a hand or even an arm without serious harm; the missing member will be regrown. But if you were to fall into a large meat-grinder—"
"I get the idea," Forrester said, feeling pale green.