The evolution of bronchial casts.

The mammalian lung provides a system of airways in which inspired air is brought into intimate contact with the blood. As well as carrying out this essential physiological function, the hollow structure of the lung has enabled investigators to produce casts of the airways, the study of which has helped in the understanding of air movements within the lung. The dependence of higher forms of life on air must have been obvious to the ancients. Anaximenes of Miletus (born c. 570 B.c.) stated that air or 'pneuma' (literally 'breath') was essential to life. The function of inspired air was open to some speculation. Plato (428-345 B.C.) stated 'as the heart might easily be raised to too high a temperature by hurtful irritation, the genii placed the lungs in its neighbourhood, which adhere to it and fill the cavity of the thorax, in order that their air vessels might moderate the great heat ofthat organ, and reduce the vessels to an exact obedience'. The indefatigable Galen (A.D. 130-199) also interested himself in the function of respiration. It may be that he was the first to have insight into its true nature for he compares it with a lamp burning in a gourd, 'When an animal inspires it is, I think, similar to a perforated gourd, but when respiration is prevented at the appropriate place on the trachea, you may compare it to a gourd unperforated and everywhere closed'. It seems that these ancient investigators did not concern themselves with the fine structure of the lungs and bronchial tree, and the record of their observations was restricted to simple drawings and woodcuts. But with the fifteenth century came a new interest in art and science which heralded the Renaissance. Anatomy derived great benefit from the more liberal ideas of the period, and the nature of the human body, demonstrated by new techniques, began to be accurately represented by the great artists of the time. One such technique was the use of casts to visualize hollow structures in the human body. Leonardo da Vinci, that 'modern biologist in the guise of a medieval artist' (F. J. Cole) injected melted wax into the ventricles of the brain to demonstrate their structure. His method, while subject to a number of defects, corrected misconceptions evident from his earlier drawings, and provides the first steps in the technique used in this laboratory to produce hollow casts of …