Is this really an important principle for humans, though? I’m not necessarily disagreeing, I’m just uncertain to what extent that selection for compactness is the main driving force here. For a burrowing animal, it’s easy to see why overall compactness could be a significant advantage. For a hippopotamus on the other hand, clearly it’s not.
Some animals do have large functional voids, for example avian air sacs. Their absence in humans obviously suggests that there was no compelling advantage in our evolutionary niche to evolving the type of structures that would be required to maintain significant air-filled voids for respiration or any other purpose. But this does not imply the converse, that there is necessarily a significant advantage to actively minimizing incidental voids because “space inside the body is too precious to waste”. The absence of voids may just be attributable to the fact that we are mostly water and somewhat squishy internally, so in the absence of any purposeful structure to maintain voids, perhaps any voids just tend to fill up.
I would guess that air voids without structural support are probably detrimental to the overall physical integrity of a body. So there’s probably also an intermediate hypothesis that it’s not necessarily that space inside the body is precious, but that voids are just bad for structural integrity.
Human have nerve pain cells almost everywhere. When anything increases size in an area where that is not intended, pain is the result.
Air in your intestines hurts. A blockage in the Eustachian tubes hurts. Edema - water retention - hurts. Swelling hurts.
Air pockets would be an invitation to pain. The surrounding matter would squeeze and compress the air, triggering the nerve cells. Simple dilation of blood vessels is a frequent cause of headaches. Notice that the exceptions people try to make, like in the ear, have bones around them to keep them from being improperly squeezed.
I admit I’m greatly simplifying anatomy. But random air pockets aren’t a structural issue. They can exist only if their value is vital and if the surrounding tissue has evolved protections. If a space existed that could be filled by valuable functions it would have long ago.
Humans actually have relatively few pain sensors inside the body as a result of how embryos evolve. Hemorrhoids that cause tremendous pain cause very little when moved a few centimetres internally. One third of the nerves in the human body supply the gut, yet most people thankfully cannot feel the continuous churning and bloating which occurs there most of the time.
You’re right, I oversimplified too much. But any number of internal pressures can cause pain or other severe problems. I can’t believe that having unfilled air pockets without a purpose wouldn’t wreak havoc on the body. Or to skip the triple negative: I believe that purposeless unprotected air pockets would wreak havoc on a body. If not pain, necessarily, then other problems.
The human body can be compared to a bagel. Basically you are a solid/liquid bagel with a hole starting at the mouth and ending at the anus.
Yes, you have lots of things projecting out from the bagel (legs, arms, ears, etc), and pockets branching from the central tube (lungs, sinuses, etc), but the body itself is either cellular, mineral, or fluids.
In the days before MRIs and CAT scans, we had pneumoencephalograms, where air is injected into the spinal canal and allowed to bubble up into the cranial ventricles, where the patient is then rotated 360 degrees is more than one direction to allow the ventricles to be seen on x-ray. By all accounts, this was extremely painful and the upside-down position in particular often led to projectile vomiting.
Possibly the best-known depiction of this procedure was in “The Exorcist”, at which time the procedure was already on its way out.
However, the facial sinuses are connected to the outside, or they’re supposed to be, anyway. So is the middle ear, via the Eustachian tubes. If they get blocked, THEN they can be really painful if the pressure can’t adjust itself.
I get that on the macroscopic level there is little to no empty space. What about the microscropic? I read somewhere that while the average human body is composed of about 10 trillion cells, it plays host to approximately 100 trillion micro-organisms. Granted many if not most are much smaller than human cells. But, wow. If true.
Well okay there may not be open garden hoses in the body, but there are spaces that can be accessed in procedures such as endoluminal endoscopy. These surgical procedures do not make any external incisions, but rather they enter the body through its natural orifices, and navigate through natural pathways. Think of entering the lung through the trachea, past the main carina, and into the bronchioles to get to a lesion in the lung. There is no incision in such a procedure. ‘Empty’ spaces are the avenues they drive through to get deeper inside.