Verse 3.6

हृदि ह्येष आत्मा । अत्रैतदेकशतं नाडीनं तासां शतं शतमेकैकस्या द्वासप्ततिर्द्वासप्ततिःप्रतिशाखानाडीसहस्राणि भवन्त्यासु व्यानश्चरति ॥ ६ ॥

hṛdi hyeṣa ātmā | atraitadekaśataṃ nāḍīnaṃ tāsāṃ śataṃ śatamekaikasyā dvāsaptatirdvāsaptatiḥpratiśākhānāḍīsahasrāṇi bhavantyāsu vyānaścarati || 6 ||

6. This atman is in the heart. Here, there are a hundred and one nerves. Every one of these has a hundred brandies; again, every one of these has seventy-two thousand sub-branches. In these, vyana moves. 

Shankara’s Commentary:

Com.—In the heart, i.e., in the akasa of the heart, enclosed within a lump of flesh of the form of a lotus, is this atman, i.e., the subtle body connected with the atman. Here, i.e., in the heart, are the chief nerves, a hundred and one in number. Every one of these chief nerves has a hundred branches. Every one of these branches has seventy-two thousand sub-branches. In these nerves, moves vyana (so called, because he is all-pervading). Vyana stays pervading the whole body through the going out from the heart everywhere within the body, as rays from the sun, especially in the joints, shoulders and vital parts. Growing active in the interim between the activities of the prana and the apana, it is able to perform deeds requiring great strength.