The aim of this paper is to depict the anatomical and physiological doctrines of the treatise entitled Περὶ πνεύματος, or De spiritu. By closely examining the contents of the treatise on its own accord, rather than through its Aristotelian or Hellenistic contexts, we attempt to overcome the aporetic and often disconnected style of the author, and to present a coherent picture of his doctrine of pneuma, its roles in the body, the anatomical infrastructure in which it acts, and its relation to the soul. We argue that the author envisions three main systems in the body: artēriai, by which external air is taken in, turned into pneuma and distributed to different parts of the body ; phlebes, by which blood is produced and distributed ; neura-cum-bones, which support the body and effect locomotion. Pneuma is shown to run through the system of artēriai, whereby it achieves vital activities such as thermoregulation, digestion and pulsation. It is also engaged in activities such as perception and locomotion, in the form of the ‘connate pneuma’ from which certain bodily parts are composed. The soul is shown to be the explanatory concept for the anatomical variety that allows the functioning of the body by means of pneuma.