It is the different components and sub-structures of the bronchial tree that masquerade as the different animals like the cow, the bullock, the tortoise, the horse, etc.
In the Mahabharata (Book 6: Bhishma Parva: Jamvu-khanda Nirmana Parva: Section IV), we have Sanjaya narrating to Dhrtarastra the fourteen species of animals:
These fourteen, O king, complete the tale of domestic and wild animals, mentioned, O lord of earth, in the Vedas, and on which the sacrifices rest.
The mobile and the immobile creatures (7 wild animals + 7 non-wild ones + 5 kinds of vegetation = 19) taken together with the five great elements make up the group of twenty four, collectively referred to as the “gayatri:”
Twenty-four in all, these are described as Gayatri (Brahma) as is well-known to all. He who knows these truly to be the sacred Gayatri possessed of every virtue, is not liable, O best of the Bharatas, to destruction in this world.
