A section of a tube is an ellipsoid, which can be a circle when the section is perpendicular to the tube.
The first and second images show tubular objects cut perpendicular to their axis. The third shows the end of one of those tubes, a hollow point, a horn. In the fourth there is only a bubble, it distorts the view like a glass hemisphere.
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Mechanical irritant dermatitis
What is inside the itching? #dermatology
The outer skin of the little lump is decapitated with a needle, revealing a small thorn. Then the rest of the spines are "combed" with the needle, scratching the skin.
At some point, it will start to bleed. Use a nail clipper or similar to remove as many raised spines (or bits of them) as you can.