Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 59


नाड्योऽस्य निरभिद्यन्त ताभ्यो लोहितमाभृतम् ।नद्यस्तत: समभवन्नुदरं निरभिद्यत ॥ ५९ ॥


nāḍyo ’sya nirabhidyantatābhyo lohitam ābhṛtamnadyas tataḥ samabhavannudaraṁ nirabhidyata


The veins of the universal body became manifested and thereafter the red corpuscles, or blood. In their wake came the rivers (the deities presiding over the veins), and then appeared an abdomen.


Blood veins are compared to rivers; when the veins were manifested in the universal form, the rivers in the various planets were also manifested. The controlling deity of the rivers is also the controlling deity of the nervous system. In Āyurvedic treatment, those who are suffering from the disease of nervous instability are recommended to take a bath by dipping into a flowing river.