The body is protected by walls of skin. The hairs on the body are compared to parks, and the highest parts of the body, like the nose and head, are compared to towers. The wrinkles and depressions on different parts of the body are compared to trenches or canals, the eyes are compared to windows, and the eyelids are compared to protective gates. The three types of metal — gold, silver and iron — represent the three modes of material nature. Gold represents goodness; silver, passion; and iron, ignorance. The body is also sometimes considered to be a bag containing three elements ( tri-dhātu ): mucus, bile and air ( kapha, pitta and vāyu ). Yasyātma-buddhiḥ kuṇape tri-dhātuke. According to Bhāgavatam (10.84.13) , one who considers this bag of mucus, bile and air to be the self is considered no better than a cow or an ass.