नियतेन्द्रिय: a man of restrained senses, परमसम्पन्न: very rich in the knowledge of scriptures, ऋत्विक् official priest, तस्य that, पतत्रिण: horse's, वपाम् marrow, उद्धृत्य having removed, शास्त्रत: according to tradition, श्रपयामास cooked.
The official priest, highly knowledgeable in scriptures and having restrained senses, removed the marrow from the horse and cooked it according to tradition.