Hippocratic treatises written in the late fifth and early fourth centuries BCE contain some of the earliest conjectures known concerning the physiological roles of the pneuma, or “breath,” that was supposed to be involved in various functions within human and animal bodies. A cross-referenced survey of these texts suggests that the contemporary theories on the subject may have gone far beyond the well-known attribution of epilepsy and other diseases to disorders in the flow of pneuma within the vessels. A pattern of co-dependent interplay between air-pneuma and fire-heat is evident among the different sources, despite disagreements of the authors on general outlook and other matters. The mutual engagement of those two elements, in turn, is found woven into elaborate mechanisms to explain, with a cause-to-effect approach, vital processes such as the regulation of body temperature through respiration, embryonic growth through morphogenetic differentiation, and even plant germination. Viewed in a historical context, these features suggest that Hippocratic speculation about pneuma may be representative of a conceptual bridging step, i.e., a stage intermediate between some seminal precedents of Presocratic thought and the more mature Aristotelian and Hellenistic theories.