The essence of the Fire principle constitutes His heart, that of the principles of waters the hip and that of the principle of the Earth the knees. Thus the other limbs too.
The Tripuṇḍraka (the three parallel lines of ash marks over the forehead) is the essence of Trinity: Brahmā, Viṣṇu and Rudra. Similarly Maheśvara has retained the esence of everything in the form of Tilaka (the small circular mark) on the forehead.
The word Bhasma means that which has controlled the essence of the whole universe. (Bha—Vṛddhi—flourishing essence. Sma—Svayam. Manyate—considers his own).
तस्मादस्य वशीकर्ता नास्तीति स शिवः स्मृतः । यथा सर्वमृगाणां च हिंसको मृगहिंसकः ७५ ।
Therefore, the one who is not subject to any control is called Shiva. Just as the hunter is considered the killer of all animals, similarly, Shiva is beyond control.