We talk about feeling liverish or having heartburn or chills on the kidney, but all these visceral images have to be invented, and our only source of information for the inventions is what we have been told. We reconstruct our insides from pictures in advertisements for patent medicines, from half-remembered school science, from pieces of offal on butchers’ slabs and all sorts of medical folklore.
These promptings may account for the otherwise inexplicable fact that the French seem to have far more trouble with their liver than the English do. It is hard to believe that this organ is so much more threatened in France than it is in England, in spite of what we have been told about their drinking habits. It seems more reasonable to assume that the French interpret their symptoms in the light of a national fantasy about the liver and unconsciously reshape their sensations in terms of this phantom organ. The English, on the other hand, are obsessed with their bowels. When an Englishman complains about constipation, you never know whether he is talking about his regularity, his lassitude, his headaches, or his depression. Once an organ gains hold on the collective imagination, its influence is almost invariably exaggerated, and a wide range of symptoms are explained in terms of it.

When an animal is large enough to need lungs, however, its bulk is so great that the physical diffusion of oxygen will not be fast enough to satisfy the insatiable demands of its working tissues. There has to be an efficient transport system capable of carrying oxygen from the lungs to every part of the living body. The evolution of the respiratory system is therefore associated with the development of an energetic heart and rapaciously absorptive blood.

Anyone who has had a serious attack of vomiting would probably find it hard to appreciate its usefulness. But it is just as indispensable as coughing and just as complicated: it is not simply intestinal action in reverse. In 1813 the French physiologist François Magendie published a pamphlet rather unpromisingly titled Mémoire sur le vomissement, in which he described an experiment which proved that vomiting is not performed by the stomach as such, but by the voluntary muscles of the diaphragm and the abdominal wall.

Theologians who use the argument from design as a proof of the existence of God frequently refer to the optical perfection of the eyeball. Ironically, this is the one part of the system where the argument from design falters, for the image projected on to the retina is so blurred and unsteady that if one tried to develop an ordinary film from it one would find it almost impossible to reconcile the smeared, hazy print with the crisp, sparkling detail of what one perceives through the eyes. In the early stages of transmission from retina to brain some of these errors are computed out: blurred contours are sharpened and a start is made on preparing the information for the part it will eventually play in conscious perception. In some ways this process resembles the image intensification which enables astro-physicists to build up pin-sharp images of planetary terrain.

It was only when the pulmonary circulation was accepted as an established fact that the results of this experiment could be recognised as significant. Meanwhile, as is so often the case, it was more convenient to make an ad hoc modification of the existing theory.