why do the tabla drums have a black spot in the middle of the drum head
Tabla The head of each drum has a central area of "tuning paste" called the syahi (lit. "ink"; a.k.a. shāī or gāb). This is constructed using multiple layers of a paste made from starch (rice or wheat) mixed with a black powder of various origins. The precise construction and shaping of this area is responsible for modification of the drum's natural overtones, resulting in the clarity of pitch (see inharmonicity) and variety of tonal possibilities unique to this instrument which has a bell-like sound. The skill required for the proper construction of this area is highly refined and is the main differentiating factor in the quality of a particular instrument.
Sarcolemma A special feature of the sarcolemma is that it invaginates into the cytoplasm of the muscle cell, forming membranous tubules radially and longitudinally within the fiber called transverse tubules (T-tubules). On either side of the transverse tubules are Terminal cisterna enlargements of smooth endoplasmic reticulum termed sarcoplasmatic reticulum (SR) in muscle. A transverse tubule surrounded by two SR cisterna are known as triads, and the contact between these structures is located at the junction of the A and I bands.