A spider carries something remarkably close to a book inside its abdomen. Book lungs are respiratory organs formed from a chamber containing dozens of extremely thin, closely stacked plates called lamellae, arranged rather like the pages of a book. Air enters through a small opening on the underside of the abdomen and circulates through the narrow spaces between them, while haemolymph flows within the lamellae. Oxygen and carbon dioxide diffuse across these extraordinarily thin surfaces. The architecture solves a geometric problem: a relatively small cavity can contain an enormous respiratory surface without requiring a correspondingly enormous organ. The spider itself solves nothing. It inherits an architecture refined across evolutionary time, one in which biological material has been arranged to multiply the interfaces across which exchange can occur.
The same structural strategy appears throughout biology at radically different scales. Human lungs branch repeatedly until hundreds of millions of alveoli provide an immense exchange surface within the chest; the small intestine uses folds, villi and microvilli to multiply absorption; mitochondrial cristae increase the membrane available for cellular respiration; cortical grey matter folds into gyri and sulci, allowing a much larger cortical sheet to occupy the finite volume of the skull. Geoffrey West’s work on biological scaling approaches the same general problem through branching networks: living systems must distribute energy and materials across increasingly large and complex bodies while remaining within severe geometric and energetic constraints. Folding, branching, stacking and subdivision are different solutions to a recurrent problem. A system does not gain capacity only by becoming larger. It can reorganise its internal structure so that vastly more productive interfaces exist within essentially the same external boundary.
The book lung consequently suggests a principle considerably larger than respiration. Internal differentiation can produce functional capacity far beyond any simple one-to-one correspondence between the amount of material present and what that material can do. Once elements are arranged so that relations themselves become productive, additional capacity appears between them: more surfaces for exchange, more pathways through a network, more possible configurations of a finite set of components. Information carries this principle still further because relational possibilities can multiply without an equivalent increase in physical extent. A finite alphabet generates an effectively unbounded field of expression because its elements can be combined, nested, repeated and recursively related. The deeper principle is not folding itself. Folding is one particularly beautiful physical instance of something more general: complex systems multiply what is possible without proportionately increasing their physical extent, generating greater functional capacity through the interfaces among their parts.
References
Brunelli, E., Rizzo, P., Guardia, A., Coscarelli, F., Sesti, S. and Tripepi, S. (2015) ‘The ultrastructure of the book lungs of the Italian trap-door spider Cteniza sp. (Araneae, Mygalomorphae, Ctenizidae)’, Arthropod Structure & Development, 44(3), pp. 228–236. doi: 10.1016/j.asd.2015.03.001.
Helander, H.F. and Fändriks, L. (2014) ‘Surface area of the digestive tract – revisited’, Scandinavian Journal of Gastroenterology, 49(6), pp. 681–689. doi: 10.3109/00365521.2014.898326.
Ochs, M., Nyengaard, J.R., Jung, A., Knudsen, L., Voigt, M., Wahlers, T., Richter, J. and Gundersen, H.J.G. (2004) ‘The number of alveoli in the human lung’, American Journal of Respiratory and Critical Care Medicine, 169(1), pp. 120–124. doi: 10.1164/rccm.200308-1107OC.
Purves, D., Augustine, G.J., Fitzpatrick, D., Katz, L.C., LaMantia, A-S., McNamara, J.O. and Williams, S.M. (eds.) (2001) Neuroscience. 2nd edn. Sunderland, MA: Sinauer Associates.
West, G. (2017) Scale: The Universal Laws of Life, Growth, and Death in Organisms, Cities, and Companies. New York: Penguin Press.
