ottokars_sandwhich:
Something flies off the area of the back of his collar, with an accompanying sound effect ("Pop!").
It's a collar stud. (Page 9, frame 2 in the standard edition, by the way...)
Before it became an easier task with washing machines, doing the laundry was an Herculean task, involving fires and boilers and hauling water, and mangles and all sorts of hard labour. You really didn't want to have to do this much more than once a week, if that, and if possible you got someone to do it for you.
One labour-saving idea was to just not to wash your shirt - or at least to reduce the times you had to. To this end various crafty methods could be deployed.
Shirts were made with detachable collars, and sometimes separate cuffs and even shirt fronts (a "dickie"). The "body" of the shirt might be made of some cheap light-weight fabric, and might be striped or patterned while the artificial front was white.
A patterned/ striped fabric "body" can still be found in some upmarket evening dress shirts today, as a sort of
hommage, even though it no longer makes them cheaper! A variation is the coloured business shirt with (integral) white collar and cuffs, which became a signature style of British politician and party-leader David Steel - almost to suggest that they could be removed when they can't.
These separate items (cuffs, fronts and collars) could be starched and laundered in bundles, and then fitted daily to make your shirt look superficially fresh. For even greater simplicity, they might even have been made of celluloid, an early form of plastic, or for "disposability" (or increased thrift), paper.
They were held in place with studs, which could also take the place of buttons for doing a shirt up (it was common for buttons to be broken going through a mangle - a device using a handle to turn heavy rollers, which squeezed water from laundry as it was passed between them, in the days before spin-dryers - so having them (buttons) detached before washing was seen as a sensible move, and detachable studs were even easier than unpicking and re-sewing actual buttons). Shirts with "grandad collars" still sometimes feature as a fashion item in shops, and often have a button hole or two, as if for use by a collar-stud.
This period also brought in the cuff-link, which is with us to this day; studs are sometimes used for very formal dress, such as evening shirts with tail-coats, or the collars worn by barristers and judges in British courts, and clergymen of some faiths.
The professor wears a high tight starched collar, so it isn't hard to imagine that it is held in place with a stud at the back. In his high state of emotion, his neck swells with a rush of blood to the head, and the force is such that his collar-stud is forced from the hole at high speed.
This isn't actually unique to Hergé - people in comics and cartoons under stress or excitement seemd to be subject to wardrobe malfunctions all the time in comics of my youth, involving exploding shirt-studs and dicky-shirt fronts which would roll up like window blinds!
