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The human body is comprised of eleven major organ systems, each composed of various structures and finely tuned to accomplish certain tasks. While certain parts are not strictly necessary for a human to live (a woman can survive without a uterus, or someone can function without one kidney), every system is needed for the body to perform properly.
Sherlock knows about all of the systems, about all of the major organs and glands and structures. There are so many ways for the body to fail; if just one organ shuts down, it can take the entire organism with it. It leaves a myriad of ways for someone to die, from something as simple as a knife in the lung to something more complex, such as a poison that blocks the absorption of oxygen in the blood, slowly depriving every tissue of the necessary molecule until the organs shut down, one by one. It is all so fascinating, how millions of years have haphazardly stitched together enough tissues that work together and only together to create fully-formed organisms.
Sherlock has seen many bodies, human and otherwise, built from the same major systems. He has held the organs in his hands and taken them apart with his scalpels, but none of them interest him as much as the systems that make up John Watson.
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John's skeleton gives him a frame 169 centimeters in height, 17 centimeters short of Sherlock's own stature. He is neither broad in the shoulder nor wide at the hip, and the rest of his skeleton is perfectly average for a man of his height and age. Most of his shape comes from other parts of his build.
His bones take something of a beating. Tackling criminals, tumbling on asphalt, blows to the head—this does not even factor in his years of rugby and anything he might have done in Afghanistan that Sherlock doesn't know about. John often comes away from encounters rubbing a shoulder or an elbow, grimacing and muttering about the bruises he will have later. He has yet to break anything, but Sherlock is sometimes concerned that eventually something will shatter—if not from a fight, then simply from the weight on his shoulders, the simple (self-imposed) stress of protecting Sherlock that may end up breaking him figuratively and literally one day.
Sherlock likes to find the areas on John's body where his skeleton is most obvious, where the bone pushes and shifts against skin: the line of his jaw, the round of his shoulders, the flexible ridge of his spine that displays individual vertebrae when he curls forward and dips into a cleft when he bends back. He wonders what these bones would feel like under his fingertips and against his lips.
His gaze flickers over John as the doctor sits in front of him now. He can see John's clavicle peeking out from under the collar of a worn t-shirt.
“You need,” John says, dragging a peroxide-soaked cotton ball across the gash on Sherlock's forearm, “to stop nearly getting yourself killed.”
“I didn't realize the suspect had another knife.” Sherlock openly stares at John's clavicle, following the sharp edge of the bone away from his throat to where it merges with his shoulder. After that, the bone disappears, packed under flesh. He can't help feeling frustrated that he cannot see it.
John sighs. His ribs lift and fall with the heavy breath. He knows, of course, knows that there is no point in repeating a tired argument.
He wraps a length of bandage around Sherlock's forearm. Sherlock watches John's hands, imagining the delicate phalanges underneath skin and muscle. The valleys between his knuckles look like they were meant to be kissed.
-----
The skeleton is rendered useless without the complex matrix of muscle overlaying it.
Most of John's shape comes from his muscles, built up from rugby and his time in the army, then maintained by the active lifestyle he leads as Sherlock's partner, giving him a strong body that is usually hidden by button-up shirts and jumpers.
Sherlock has never been particularly interested in or aroused by bodies in this way, male or female—skinny, fat, lean, built, all of it is subjective and dull unless those bodies have something interesting to tell him. He admits, though, a certain appreciation for the relationship of form and function, which he repeats to himself when John stretches for a shelf in the kitchen and his jumper rides up, revealing the line of the strong abdominal oblique above his hip.
And when John lifts a heavy box that Sherlock left carelessly in the lounge, making the deltoids of his shoulders and the biceps and triceps of his upper arms flex under the thin cotton of an old t-shirt.
And when John wears an old pair of jeans that he refuses to admit are slightly too tight, so the denim curves lovingly over the tops of the gastrocnemius muscles in his calves when he sits and clings to both gluteus muscles when he stands.
And when he tilts his head to crack his neck, straining the sternocleidomastoids and the trapezius and the wiry tendons until Sherlock thinks the skin could simply split.
And essentially any other time he can observe the dormant strength of John's body in action, where it sits just beneath the surface of his skin, hiding the truth of the man under an unassuming exterior. John appears to many as outwardly harmless and weak, but Sherlock knows the truth of it.
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The endocrine system is invisible and less interesting, but it is just as vital.
The pancreas releases insulin and glucagon to regulate glucose levels. The hypothalamus manages a range of mood-balancing chemicals. Adrenal glands (the adrenal medulla, specifically) kick into gear whenever John has to chase Sherlock across London or draw his gun, pumping adrenalin into his blood, and a healthy dose of endorphins alongside that tell his brain that this is what he likes.
Sherlock thinks about John's not-quite-addiction to excitement and adrenaline that keeps him at Sherlock's side. He decides he really is quite fond of the endocrine system after all.
-----
Sherlock is almost always aware of John's breathing. He has, on at least two occasions, demanded that John shut up and stop breathing so that he can think, and both times realized that the room was empty without the background noise of another living human being.
John's respiratory tract is perfectly healthy; unlike Sherlock, he has never succumbed to a smoking habit or exposed himself to enough chemicals to fall ill. Sherlock wants to open John's chest and see, to watch his lungs inflate with each breath and deflate, to view the diaphragm as it flexes and contracts, to examine the larynx that somehow produces a violent shout and a loving whisper and an entire range of voices in between.
Sometimes, it worries him. After a long chase across London's rooftops or a mad dash through the streets, John always doubles over, hands on his knees, wheezing for air. He always looks up and grins, though, even as he huffs through his teeth, and Sherlock can only smile back.
John is breathing heavily, but he isn't smiling as he throws his arms around Sherlock's middle and squeezes.
“Stupid git,” John hisses into Sherlock's shoulder. “Do you have any idea what I thought—when I heard the gunshots—I thought you were dead!”
Sherlock is silent. He stares at the blood trickling from John's temple, evidence of the strike of a pistol. He had not accounted for the murderer having accomplices; his mistake might have killed one of the only people he might mourn. He wants to be dismissive of John's fears, but to do so would mean denying his own.
John's body is heaving. His lungs expand and collapse rapidly, each breath pushing his chest into Sherlock's. Sherlock wraps his arms around him and realizes they're breathing in tandem, and his own breaths are just as ragged.
-----
While Sherlock has learned to exert control over the whims of his body, John has not. This often leads to Sherlock compensating for his flatmate's needs, meaning leaving John at the flat so he can sleep for two hours or ducking into a restaurant for a meal he knows he will not have a chance to finish.
The process of digestion is not an altogether pleasant one, or even a highly efficient one (as far as Sherlock is concerned, it is a system that could stand to be more streamlined), but if nothing else, it can provide information. He remembers one case in which the murderer's alibi was discredited by the presence of partially-digested food in the dead woman's stomach—had she been alive at the time he claimed to be with her, late in the evening, her stomach would have been empty of the lunch she had eaten.
Sherlock does not care to think of cutting open John's stomach in the event of the man's death, so he completely dismisses the memory of that case as he sits across the table from John at Angelo's. He alternates between looking out the window, at the flickering candle on the table (John has given up protesting the candle, Sherlock notes), and at John.
“You really should eat,” John says, pointedly pushing a small basket of rolls across the table with the tip of his butter knife. “It's been a couple of days. Digestion might slow you down, but passing out from hunger would slow you down even more.”
Sherlock says nothing. John gives an exaggerated sigh. Eventually, Sherlock picks up a roll and sets it down on the table to pick at. He puts one piece in his mouth (amylase in the saliva begins breaking down carbohydrates before they reach the stomach) and barely tastes it (the arrangement of taste buds on the tongue is as unique between individuals as a fingerprint, and the concept of a “taste map” has been disproved for decades) as he swallows. The rest of the roll goes untouched.
He watches John eat across the table—the working of his jaw, the bob of his Adam's apple with each swallow. Sherlock's gaze automatically trails down along the bit of exposed throat, down John's chest to just below his ribs. The esophagus automatically flexes in a movement called peristalsis, pushing food down, behind the trachea and to the stomach, where it's broken down and dissolved beyond recognition by acids and enzymes. Nutrients are taken from the resulting mess that, in this case, was once a surprisingly decent fettuccine—amino acids and vitamins, molecules that eventually will be used to make up one John Watson.
John licks a bit of sauce from his lips and Sherlock's eyes immediately snap up to the movement, just in time to see the tip of a pink tongue swipe across a thin upper lip.
Perhaps stopping for meals is not as tedious as he thought.
-----
It occurs to Sherlock one day to wonder if thinking of someone like this is, as has become code between him and John, “a bit not good.”
He decides he does not care. Besides, John is a doctor. He would understand, and he is constantly thinking about Sherlock's body, if the nagging about food and sleep and everything else is any sort of indication.
What if John thinks about Sherlock as extensively as Sherlock thinks about him?
-----
Sherlock cannot fathom how the heart, an organ built specifically for pumping blood, came to be associated with love and emotion. If people wanted to associate parts of the body with that, they would be much better off defaulting back to the hypothalamus, which had everything to do with emotions that the heart and blood vessels did not.
Then again, the pulse is a remarkable indicator of health and mood.
“A lot of people take pulses wrong,” John remarks, his eyes fixated on the telly, which is droning some (vastly dramatized and completely incorrect) medical show. When Sherlock looks up from the sofa, curious, John continues. “When they try to do it on the wrist. You have to do it right or you'll just end up taking your own pulse instead.”
Sherlock knows this, of course. He also knows there are multiple points on the human body where the pulse can be taken, though the neck and wrist are the most practical. If given the chance, he would take his time finding each one on John and press his lips and tongue to them, feeling the life flowing under the thin layers of skin.
“How is it done, then?” he asks, though he already knows. The doctor beckons him over and holds Sherlock's arm delicately in one hand, as though he will break (he might, if it is John doing the breaking). He presses the first two fingers of his other hand to the inside of Sherlock's wrist.
“I've seen people take pulses with their thumb before,” he says. “Don't know where they learn it.”
Sherlock is silent, fighting to control his breathing, though this simple touch should not be enough to excite him. It is.
He would start at the top, as one should, with the temporal pulse at the side of John's head, then move down, past the strong pulse of his carotid artery in his neck, then through the brachial and radial points of his arm and wrist. Then he would go across, through the femoral, popliteal, and pedal arteries from John's thigh, to his knee, to the top of his foot. The last he would visit would be the apical, the apex: the strong beat of John's heart in his chest.
John raises his eyebrows slightly. “Pulse is a bit fast,” he remarks, then smiles playfully. “Nervous about something?”
Sherlock swallows and says nothing. His pulse seems to rise whenever John smiles at him or touches him or even lets his gaze linger for more than a second or two. He wonders if the same thing happens to John.
He reaches for John's hand to find out, but John frowns at him and pulls his hand away.
He thinks he understands, then, why the heart is associated with emotion, when he feels a cold, heavy sensation in the center of his chest.
-----
Sherlock's interest in John's reproductive system (besides obvious, crass desires) is limited to his loathing of whatever it is that makes John seek out the company of women.
-----
The integumentary system, consisting of only one real organ (the skin) as well as the hair and nails, is the system of bodily protection. It is also the system that tells stories.
Indents in the skin of the ring finger betray an adulterer's lifestyle. Bitten fingernails showcase anxious habits. Hairstyles can reveal the personalities of those who wear them. Sherlock can deduce a great many things about his John from his physical appearance, which makes the system one of his favorites.
John's military habits show in his hair and his nails—both are kept short and clean. The wrinkles around his eyes and mouth are more pronounced after a particularly stressful day at the surgery. His hands (those brilliant, dextrous doctor's hands) have tiny marks and scars from his former life as an army doctor, and Sherlock wants to know the stories behind every single one. His favorite, however, is the scar in John's shoulder, the puckered round mark where the bullet pierced flesh and muscle and sent him back to London.
John is in a rather good mood today and humors Sherlock when he says he has an experiment. He runs his hand through John's hair, slowly, letting each strand bend back under his hand and spring forward again. As he thought, the few gray strands that pepper John's blond coloring are coarser than their fellows. He traps one between his thumb and forefinger and wonders what caused the color to leech away. Is it age? John has yet to hit forty, but people go gray at different ages; Lestrade was not yet fifty and most of his color was gone. Sherlock wonders, with a pang of (undeserved) guilt, if it is stress. Work at the surgery is boring and John is never with a girlfriend long enough for a woman to stress him significantly, so that would leave Sherlock as the cause. But no, John enjoys the work almost as much as Sherlock does, and if it were that awful, he would have left by now.
Amazing how John tells his life's story and inspires fear in Sherlock without uttering a single word.
“Find something interesting?” John asks, not lifting his eyes from his book. Sherlock blinks; his hand has been cradling the back of John's head for thirty full seconds.
“Yes,” he says, but when John looks up at him, he does not elaborate, because if he begins listing the things he finds interesting about John, he fears he will never stop talking.
-----
In the interest of keeping his image of John unsullied and ideal, Sherlock specifically spares no significant thought to the man's excretory system. The only time he does is when a suspect punches John in the kidney so hard he doubles over in pain, and Sherlock spends the rest of the night fretting so much that John yells at Sherlock to leave him alone until he starts pissing blood. (He never did, meaning an entire night was wasted on unnecessary fear. John seemed amused by the idea that the supposed sociopath could worry, though.)
-----
The lymphatic-immune system is sometimes referred to as such because, while the lymphatic system is the actual structure of glands and lymphatic vessels and nodes, the cells and the parts of the immune system responsible for them are very often the same.
Sherlock thinks they should rethink the “immune” part of the name, considering John's so-called immune system has proven itself incapable of preventing common influenza.
The man looks like death—and feels worse than it, by his claim, which is ridiculous, because one does not feel anything when dead. He is shivering through the throes of a fever despite Sherlock's fluffiest duvet—which he may have to forgo washing and simply burn—and downing ibuprofen tablets as though they were candy; a mild fever could run its course, as it is the body's way of killing foreign bacteria and viruses, but John's body does not know when to quit and his 39-degree fever cannot go unchecked. His throat is sore (swollen glands), his muscles weak (deprived of nutrition and the body's energy is diverted to the destruction of the virus), his stomach unsettled (body may or may not attempt to purge itself even though vomiting would not solve the problem).
His eyes dart under his lids and he mumbles, caught up in a series of fever-induced dreams that Sherlock cannot begin to decipher. He does not seem terrified by memories of Afghanistan, so Sherlock does not wake him.
John became irate earlier when Sherlock tried to pick his brain for information about influenza, so instead the detective sits with his laptop and five tabs open with Google results. Fascinating how evolution (and human stupidity) has led to multiple strains of the virus, how the virus injects its own DNA into human cells and dupes them into replicating the virus; worrying how John's fever is apparently straddling the line between “manageable” and “needs hospitalization.”
Sherlock looks up more information on the body's defenses and reads about leukocytes. There are actually multiple kinds of white blood cells, specialized for various purposes; if Sherlock is correct, and he normally is, John's body would currently be loaded up with lymphocytes, monocytes, and macrophages in particular, each with different purposes.
Like little soldiers, Sherlock thinks idly. Lymphocytes as generals, coordinating responses and leading the fight; monocytes, the scouts, dispatched to various part of the body for specialized tasks; macrophages, the fighters who engage in combat, though he doubts Her Majesty's Army kills via phagocytosis.
Microscopic soldiers for the soldier. Send in the cavalry.
He frowns. Metaphor is as much his area as girls are.
John blinks awake and groans, pained. As he tries to sit up, his body gives a heave, and Sherlock is immediately on his feet, shoving a plastic-lined garbage bin in front of the sofa and guiding John over it. After John finishes, Sherlock silently whisks the mess away and goes to the kitchen, where an already-boiled kettle and a mug with a tea bag wait.
John looks surprised when Sherlock tentatively offers the cup of tea.
“For your throat,” Sherlock explains. “And you're dehydrated.” Not that tea is the best thing for dehydration or malnutrition, but forcing much else down John's throat seems like asking for trouble.
John smiles faintly. “Thanks,” he rasps, taking the tea between both shaking hands. He stares down at the liquid, as though lifting his head to address his flatmate is beyond his ability. “You don't have to do this, you know. I know you're bored, and you'll probably just get sick, too.”
Sherlock says nothing, instead dragging the duvet over John's shoulders when he sees him trembling. He remembers something his mother would do when he was sick, leans in, and gently presses a kiss to the side of John's head.
John seems amused. “You can't actually kiss the flu better, Sherlock,” he says.
He's smiling, though, for the first time in two days, so Sherlock fancies that maybe he can.
-----
Sherlock loves and hates John's nervous system.
He hates it because it is broken. The psychosomatic limp is not real. The tremor in John's left hand has no purpose and does not make conventional sense; the average person does not tremble because they lack danger.
He loves it because without it, John would not be John. He would not have his army-trained instincts, the reflexive fight-or-flight mechanisms of the somatic subsystem that saved Sherlock's life on only the second night they knew each other and have saved them both countless times since. Without the autonomic functions, he would not blink or breathe; his heart would not beat in his chest. A lack of nerves would mean he would not feel pain or pleasure and would not react to stimuli in ways that still sometimes surprise Sherlock to this day. At the center of it all lies the most interesting part: John's confusing, brilliant (in its own way) brain.
John's somatic nervous system seems to run at peak efficiency at all times, but there is a two-second delay between the moment Sherlock kisses him and the moment when he reacts.
John shoves him away.
Sherlock stares at him. John's fist is curled at his side—he would have punched Sherlock, then (instinct) but refrained (managed to override the reflex, not an easy feat, takes a significant amount of self-control because the action of a reflex does not even go to the brain). A flush has risen faintly in his cheeks (automatic, another deed done by the autonomic system, a reflex to embarrassment, why is he not instead enraged?) His hand does not shake.
John takes a deep breath. His expression is unreadable. “What was that?” he asks, his voice carefully neutral.
Sherlock tries to fight first. The best defense is a good offense, and for him, both of those are wit and sarcasm. “A kiss, obviously,” he says stiffly.
John huffs. “Obviously,” he repeats. “I got that, yeah.”
Fighting is failing. Perhaps he should have fled. He starts to turn, but John catches his arm.
“No,” he says. “You can't walk off after that.”
Sherlock looks back at him, trying to maintain a neutral face.
John takes another deep breath. “What was that?” he says again. “Because this can go a few ways, Sherlock, and I swear, if this was one of your damn experiments . . .”
Sherlock thinks about it. He thinks about telling John precisely that, because then he can deny any other motives. John never has to know that Sherlock is obsessed with everything about him, from the way that he speaks to the shape of his bones to the hairs on his head.
“It isn't,” he says instead. The words are not a reflex, but the decision to speak appears to have bypassed his brain, anyway.
There are certain moments when Sherlock can almost see John thinking. He can imagine the elevated brain activity that would glow on an MRI scan, the firing of neurons and the jump of hormones and electrical pulses through the synapses as the brain rapidly processes new information and formulates the proper response.
Slowly, a smile spreads across John's face, and he moves. A hand wraps around the back of Sherlock's neck and he is being bent down to John's level.
There are higher concentrations of nerve endings in the fingertips and the lips, allowing for more information to be gained from tactile sensations. When John kisses him, and Sherlock hesitantly lets his hands slide down across a wool jumper, feeling the warmth of skin through two layers of fabric, he finds he has never been so grateful for that fact. The heat seems to spread from every point of contact until his body is burning with it, and he knows with absolute certainty that John feels the same.
-----
Sherlock wants to know John's body as intimately as he knows his own. He takes his time, grazing teeth over a sharp collarbone, watching a heightened pulse jump in a carotid artery, kissing the pale roughness of scar tissue, listening to soft gasps and growled pleas deep in the throat, sliding his hands across inches of exposed skin and murmuring the names of bones and organs hidden underneath. Then John does something impossible with the eight clever muscles of his tongue and Sherlock's focus is utterly destroyed.
There is time now, though, for him to appreciate everything about his John. Every system, every part, every meager little molecule.