Śivasaṃhitā 1.91

The body as brahmāṇḍa—microcosm that replicates the macrocosm. Everything that exists in the universe exists in the body: the five elements, the divinities, the worlds. This vision is not merely poetic but the basis of Tantric practice: working with the body is working with the entire universe. The yogi need not go anywhere—the field of work is the body itself.

Prameyatva (the condition of being an object of knowledge, of being knowable) describes the relationship between the conscious subject and the objects of the world. Everything that can be known is prameya. Brahmāṇḍa (the egg of Brahmā, the body as microcosm) is one of the richest concepts in Tantric cosmology: the human body contains in miniature all the principles of the universe. The spinal cord is meru, the heart is the sun, the brain is the moon.

The doctrine of the body as microcosm (brahmāṇḍa) is central in Tantric yoga and has ramifications in Indian alchemy (rasāyana), in Āyurvedic medicine, and in astrology. In the Śivasaṃhitā, this doctrine grounds the following chapters: the practices of prāṇāyāma, mudrā, and dhyāna are effective precisely because the body contains all cosmic principles and can be worked as a complete universe.