Katha-2-1-8-araṇyornihito

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

Again, the sacrificial fire lodged in the upper arani and the lower arani, the eater of all offerings and the fire within the body; which are well-maintained by the ritviks and yôgins, respectively, as the foetus is well-maintained in the world by pregnant women, by means of food, drink, etc., not condemned. Moreover, fit to be worshipped, i.e., eulogised and adored every day by those who follow Karma and by those who follow yôga, in sacrifices and at heart; jâgrivadbhih, who are by nature on the alert, i.e., watchful. Havishmadbhih, by those having offerings such as ghee, etc., and by those having contemplation and meditation. This verily is that—that Brahman now treated of.

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda