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 structures 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; bones and neura, which support the body and effect locomotion. Pneuma is shown to run through the system of artēriai, whereby it performs 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,”which, we propose, is a component of bodily parts. The author connects pneuma very closely with soul, and although he is familiar with Aristotle’s doctrine of the soul, he does not see to embrace it.