Disclaimer: I don't own Supernatural or any of the characters.

A/N: This fic is just the product of the mind of a medical student, who had to learn all the muscles, nerves, arteries and veins of the human body in three weeks. Call it therapy, :P. Hope you enjoy it!

Anatomy

"I hate this," Dean snapped. "I really, really hate this." He slammed the door shut and stalked over to Castiel, who was standing awkwardly besides his bed. "Turn around." Castiel complied and Dean started to undo the knot to the sling that was holding his arm. "You shouldn't get hurt."

"I'm fine, Dean."

Dean pressed his lips together and the white cloth of the sling became undone and slowly slid to the floor, while Castiel did his best to keep his arm still. "You broke your collar bone, Cas. You're not fine."

"It'll heal. The doctor said it'll heal." Castiel sat himself down onto the bed. He kept his left hand on the broken bone to immobilise it as much as possible, yet the movement still made him wince.

"You're in pain," Dean said angrily, dropping to his knees to untie Castiel's shoelaces. "You were an angel. This isn't supposed to happen to you."

"I'm not an angel anymore." The quiet regret in his voice took the edge off Dean's anger and made him pause. He absentmindedly picked at a piece of dirt already deeply ingrained in Castiel's jeans, then set about gently removing his shoes. Castiel, in the meantime, tried to undo the buttons of his shirt with one hand. Dean quickly took over and helped him out of it.

"Thank you."

"No problem," Dean sighed wearily, discarding the shirt on the floor. "You should get some rest. Do you want more painkillers?"

"No, I'm fine." Castiel shook his head dismissively when Dean offered him the bottle, ignoring the scrutinizing look Dean sent him. Then he tried to lower himself slowly down onto the bed, but gasped halfway through the motion and quickly sat up again.

"Will you stop saying you're fine?" Dean said, annoyed, putting the painkillers on the nightstand just in case. "You're clearly not and you're damn lucky you got away with just a broken bone."

"I'm sorry." Castiel's hands were in his lap, his shoulders hunched. He fidgeted with his fingers and Dean suddenly realised how very strange this had to be for Castiel, sitting shirtless and injured in front of him, more vulnerable and dependant than he'd ever been in his previous life.

Dean immediately felt guilty. "Not your fault. Don't mind me, alright? Here, let me help lie you down." He put one hand on Castiel's neck, the other on his back and carefully guided him down onto the bed. The action reminded him of putting little Sammy down in his crib back when he'd been an infant. As soon as Castiel was comfortable, Dean quickly removed his hands and turned around. His baby brother wasn't a memory he wished to revisit. He grabbed Castiel's shirt from the floor and attempted to fold it into something relatively neat.

"Clavicula," Castiel said suddenly and Dean put the shirt down again to stare at his friend in confusion.

"Excuse me?"

"Clavicula," Castiel repeated, as Dean sat down on the side of his bed. "That's the Latin name for the collar bone. It translates to 'little key', referring to the motion the bone makes whenever the arm is rotated." He looked at Dean hopefully, as if he had something to apologize for and spewing random trivia was his first step in making amends.

"I didn't know that," Dean said softly. Castiel had removed his hand from his collar bone and Dean saw how his injured shoulder sat slightly lower than the healthy one. Without thinking about it, he reached out and carefully placed two fingers on the bone, gently running them along the skin, his touch feather light. Castiel didn't flinch. Every other person would have felt the need to protect themselves from possible pain caused by the jarring of a broken bone, but maybe Castiel was not human enough to have gained such reflexes. Perhaps he simply trusted Dean too much and couldn't believe he'd ever hurt him. For the second time that night, Dean was reminded of his brother.

"Angels consider human beings purely as a whole," Castiel said, following the way Dean's fingers lightly traced the contours of his collar bone, going from the point where it attached to his sternum and back again towards his shoulder. "It was humans who disassembled the body and gave names to the different parts."

"That's because we destroy everything we touch." Dean was surprised by the bitterness in his own voice. "We just have to tear it all apart."

Castiel focused on Dean and there was the hint of a rebuke in his voice. "You did it to advance the art of medicine. To heal people and to put an end to pain."

"Playing for God," Dean scoffed, avoiding his eyes. "Wouldn't that be considered pride or something?"

Castiel frowned. "Why would God condemn the wish to ease suffering?"

It was a genuine question and Dean squeezed his eyes shut for a moment. This was wrong. He and Castiel shouldn't be discussing theological matters as equals. Castiel had been an angel. It wasn't right that they were on the same level now. It wasn't right that Castiel had to ask these questions in the first place. He ran his fingers along the broken collar bone a final time, before resting them on Castiel's shoulder.

"Musculus deltoideus," Castiel supplied. "Shoulder muscle, shaped like the Greek letter 'delta'."

"You know the names of all the parts?"

Castiel nodded. "Yes, although I am the only angel who does. My brothers and sisters would ridicule my human knowledge. They believe it unworthy of a celestial."

"Why do you have it then?" Dean asked, his fingers exploring the muscle on Castiel's shoulder.

"I rebuilt you," Castiel replied, as if it was the most obvious fact in the world. Then he continued, unaware that Dean's breath had stuck in his throat. "The musculus deltoideus inserts on the humerus, the bone of the upper arm."

Dean moved lower to find the point he was referring to.

"Musculus biceps brachii." The syllables rolled smoothly from Castiel's tongue. His pronunciation lacked the American accent Sam, Dean and Bobby were unable to shed when speaking Latin. It was pure, inviolate, from a time long before the church and science had claimed the language as their own. Dean didn't really want him to stop speaking. "Beneath it lies the musculus brachialis."

Castiel's arm was resting on the bed with the palm upwards as Dean passed his elbow, his fingertips caressing the smooth skin.

"Two bones of the forearm," Castiel continued. "Radius and ulna. The ulna is on the medial side."

"Here?" Dean indicated the part of Castiel's arm that was continuous with his little finger and closest to his chest.

"Yes. There are many muscles in the forearm. Your hand is on the musculus flexor carpi ulnaris. It runs down to the base of my hand."

Dean followed its course and lightly grabbed hold of Castiel's fingers. "The tendons of the musculus flexor digitorum profundus, inserting on the phalanges distales."

Normally, Dean would've had to comment on the ridiculousness of the lengthy names Castiel was pronouncing with such care, but there was nothing at all humorous about them right now. Castiel had stopped speaking as Dean had stopped moving and now seemed to wait patiently for what was going to happen next. Dean tentatively placed his fingers on Castiel's chest, inches below his collar bone.

"Musculus pectoralis major," Castiel immediately supplied and Dean thought he could hear a faint note of relief in his voice, as if Castiel didn't want him to abandon what he was doing. "The largest muscle of the chest. It originates from the clavicula, sternum and several costae."

"Costae? Are those the ribs?" Dean asked, moving down and tracing the lowest one. Castiel shivered as Dean's fingers reached the sensitive skin on his side.

"Yes," He answered, pressing his right arm closer to his chest to force Dean away from ticklish area.

Dean smirked and moved to his stomach.

"Musculus obliquus externus abdominis," Castiel said, relaxing again and lazily following Dean's fingers with his eyes. They came to a halt at the top of his hip bone. "Crista iliaca."

"You're too damn skinny, Cas," Dean rebuked him gently. "It's way too easy to feel all this stuff." He prodded at the protruding bone.

Castiel averted his eyes. "Sorry."

"It's okay," Dean said quickly, brushing his thumb gently over Castiel's stomach. "Remind me to pick up some things you like in the next few days, alright?"

"Alright," Castiel echoed. He looked up at Dean expectantly and Dean realised he'd reached Castiel's jeans. After momentary hesitation, he began to undo the belt, followed by the zipper. Castiel attempted to lift his hips to allow Dean to slide them off, but gasped at the pain in his collar bone.

"Hey, hey, easy!" Dean said sternly, pressing him back down onto the bed. "I've got this. Don't hurt yourself." He carefully pulled the jeans down over Castiel's knees and past his feet, before dropping them unceremoniously on the floor. Castiel's socks quickly joined them, leaving the former angel in just his generic black boxers.

Dean didn't immediately resume his exploration of Castiel's anatomy. He considered simply covering his friend with a blanket, before leaving him to sleep. Castiel had other plans. He grabbed Dean's hand and placed it back on his hip. "Crista iliaca," he repeated with emphasis, his blue eyes fixed on Dean's face.

Dean sighed. "Fine." He quickly moved his hand down past the boxers, resting it on Castiel's thigh.

"Femur," Castiel spoke up. "And the musculus quadriceps femoris. The largest muscle of the thigh, composed of the musculus rectus femoris, vastus medialis, vastus lateralis and vastus intermedius."

Dean moved his fingers to the inside of Castiel's leg. When the motion wasn't accompanied by any commentary, he looked up. Castiel's mouth was opened slightly and he was staring at Dean. "Musculus … musculus sartorius," he eventually uttered, a little breathless.

Dean swallowed. It'd be so easy to slide his touch up to the angel's groin. He realized that he could and that Castiel wouldn't stop him. Castiel would never stop Dean, even if he wanted to, and that thought scared him. But when he make an attempt to withdraw completely, Castiel raised his left hand and grabbed a hold of Dean's fingers again, attempting to move them up. Dean stopped him, diverting the course of Castiel's hand in order to put it back down on the bed. "No. I'm not gonna do that tonight. I'm not taking advantage of you."

"You're not," Castiel protested and Dean noticed his pupils were slightly dilated.

"I am. You're injured, you're tired and you almost died today. Not now, Cas. Please."

"Okay," Castiel agreed softly and he rested his head on the pillow again, staring up at the ceiling.

"Okay," Dean repeated, moving his fingers down and tapping the bone in Castiel's knee. "What's this?"

"Patella," Castiel said. Dean was glad to hear he didn't sound regretful. He ran his hand down Castiel's lower leg, feeling the soft hairs on his shin. "Two bones, the tibia and fibula," Castiel continued. "Musculus tibialis anterior."

Dean went lower and reached his foot, resting his hand on Castiel's toes, like he'd done with his fingers. He wondered whether Castiel had ever looked at Jimmy Novak's body with the same attention Dean was giving it now. Whether he'd ever made it fully his own after falling from grace and being forced to inhabit it for the remainder of a mortal life. "Musculus extensor digitorum longus," Castiel continued, ignorant of Dean's musings. "Supplied by the nervus peroneus fibularis profundus."

Having reached the end of Castiel's leg, Dean removed his hand and placed it once more with the palm flat on Castiel's chest, this time on the left side.

"Cor," Castiel whispered and Dean felt the steady beating underneath his fingers. "My heart." There was a silence and Dean almost imagined he could hear the sounds of the valves closing. "The Greeks thought the soul rested there."

"What do you think?"

"I think," Castiel said carefully. "that not everything can be named."

Dean nodded. He kept his hand on Castiel's heart and tried not to consider how very intimate this was. Castiel closed his eyes. Dean noticed how he was breathing shallowly and knew from experience that deep breaths would be painful in his current condition. He still hated seeing his friend this fragile, but forced himself to concentrate on the unwavering heartbeat. Cas was fine. He was alive, he was warm, he was fine.

Dean waited fifteen minutes, then slowly took his hand away and grabbed a blanket from the foot of the bed. Gently, he covered Castiel with it.

He was almost at the door when he heard Castiel speak again, his voice heavy with sleep. "Do you understand now?"

"Understand what, Cas?"

"Why rebuilding you has been the most important thing I've ever done?"

Dean hesitated, his hand already on the door handle. He looked back to the bed. Castiel's eyes were still closed. "Yes. Yes, I do."

The End