So many things in life owe their beautiful complexity to the simple process of repetitive splitting. Ever efficient, nature designed the bronchial structure and the tree to achieve the same thing - to maximise surface and exposure to optimise the chance of survival. It reminds me of a beautiful line I read once in a medical textbook - 'when the lungs are obstructed, the patient snatches at inspiration'.

This image reminds me of so much that I love about that period of history when we were discovering the world. It reminds me of the delicate ornateness of Vesalius' early anatomical drawings, of the urge to describe the world and the skill of the illustrator who subordinates artistry to recording the thing itself.

Old Masters started with a dark canvas and painted the light in. These days, we start with gesso and spend our time chasing the light away. Here, Moffit catches depth, weight, reflected light and mortality floating in a dark canvas.

It's the cross. Take the cross away and this is just a caricature sketched on a napkin. The cross floats so perfectly, falling exactly where it should, with the weight it should. It took real skill to put that cross there, and I bet the artist never gave it a moment's thought.

Chinese artist Chen Hong recently completed a residence at the Fitzwilliam Museum in Cambridge. He specialises, indeed is acknowledged as a world-leading proponent in the drawing of fish. Carp, to be precise. Watching a film of his drawing, time after time, his practised hand would define with a single brushstroke the sinuous spine of a carp. Something about the confidence and fluidity of this Japanese artist's work reminds me of watching Chen Hong painting those iridescent fish.

Lines are everywhere in our lives. They describe, they proscribe. They set boundaries and give directions. It reminds me on a Parag Khanna's TED talk about how much better the world would be if we ignored the straight lines of geopolitical boundaries and instead respected the curvy lines set by rivers, mountains and chasms in the earth.

I love this for one reason, and one reason alone. it is the single thick line which swoops downward to define Leo Bensemann's back. The rest of the drawing I can take or leave, but that line makes me happy.

This probably grew up to become a masterpiece, but I prefer this, in its raw state. It's like being able to see the mathematics behind the world.

I volunteered very briefly at No. 2 Willow Road, a Modernist house in Hampstead in London. There was a spiral staircase just like this leading down to the custodian's flat. One night, I sat in an Eames chair and sketched the staircase. It was a terrible, terrible drawing, so I like to pretend that this was the one I drew instead.

Which line do you think started this picture? How certain does an artist need to be of his composition when his pencil makes that first mark which commits the whole structure of the piece? I think the first line was the one describing the curve of the girl's back, which holds the rest of the piece in place.

This started life as a blank piece of paper - Grierson has used line and perspective to describe depth so skilfully that you can easily forget the craft of it.