Early thoughts on the biology of man(prasnopanisad-1):
The sky, wind, fire, water and earth are formed the senses The speech , mind, vision, hearing are senses But the senses need the life force to function The life force is necessary for the humans, angels and the animals. It is activated from the womb itself The life forces are the messengers between the god and man The life force emanates from the absolute It centres itself in the mind first and then spreads out to all parts of body It takes food,digests it and takes to all organs It is in the heart and through the arteries it reaches all organs and limbs The life force spreads upwards with the help of fire. The fire is positioned in the eyes The earth, sky and wind are other forces which influences the life The fire takes the man to heaven,hell or back to the earth depending upon his personality after death.