"Bharadwaja said, 'How does bodily fire or heat, entering the body,
reside there? How also does the wind, obtaining space for itself, cause
the body to move and exert itself?'

"Bhrigu said, 'I shall, O regenerate one, speak to thee of the course in
which the wind moves, and how, O sinless one, that mighty element causes
the bodies of living creatures to move and exert themselves. Heat resides
within the head (brain) and protects the body (from perishing). The wind
or breath called Prana, residing within the head and the heat that is
there, cause all kinds of exertion. That Prana is the living creature,
the universal soul, the eternal Being, and the Mind, Intellect, and
Consciousness of all living creatures, as also all the objects of the
senses.K.P. Singha wrongly renders this verse. The Burdwan translator is right. Thus the living creature is, in every respect, caused by
Prana to move about and exert. Them in consequence of the other breath
called Samana, every one of the senses is made to act as it does. The
breath called Apana, having recourse to the heat that is in the urethra
and the abdominal intestines, moves, engaged in carrying out urine and
faeces. That single breath which operates in these three, is called Udana
by those that are conversant with science. That breath which operates,
residing in all the joints of men's bodies, is called Vyana. There is
heat in the bodies of living creatures which is circulated all over the
system by the breath Samana. Residing thus in the body, that breath
operates upon the different kinds of watery and other elementary
substances and all bad humours. That heat, residing between Apana and
Prana, in the region of the navel, operates, with the aid of those two
breaths, in digesting all food that is taken by a living creature. There
is a duct beginning from the mouth down to the anal canal. Its extremity
is called the anus. From this main duct numerous subsidiary ones branch
out in the bodies of all living creatures.Both the Bengal and the Bombay texts read bhutani. The correct reading, however, appears to be bhutanam. In consequence of the
rush of the several breaths named above (through these ducts), those
breaths mingle together. The heat (that dwells in Prana) is called
Ushman. It is this heat that causes digestion in all creatures possessed
of bodies. The breath called Prana, the bearer of a current of heat,
descends (from the head) downwards to the extremity of the anal canal and
thence is sent upwards once more. Coming back to its seat in the head, it
once more sends down the heat it bears. Below the navel is the region of
digested matter. Above it is that for the food which is taken. In the
navel are all the forces of life that sustain the body. Urged by the ten
kinds of breaths having Prana for their first, the ducts (already
mentioned), branching out from the heart, convey the liquid juices that
food yields, upwards, downwards, and in transverse directions.The word for duct is Srotas. It may also be rendered 'channel.' Very like the principal artery or aorta. The
main duct leading from the mouth to the anus is the path by which yogins,
vanquishers of fatigue, of perfect equanimity in joy and sorrow, and
possessed of great patience, succeed in attaining to Brahma by holding
the soul within the brain.Notwithstanding much that is crude anatomy and crude physiology in these sections, it is evident, however, that certain glimpses of truth were perceived by the Rishis of ancient times. Verse 15 shows that the great discovery of Harvey in modern times was known in ancient India. Even thus is heat panted in the breaths
called Prana and Apana and others, of all embodied creatures. That heat
is always burning there like a fire placed in any (visible) vessel.'