Split the Lark—and you'll find the Music—
Bulb after Bulb, in Silver rolled—
Scantily dealt to the Summer Morning
Saved for your Ear, when Lutes be old —
Loose the Flood—you shall find it patent—
Bulb is a generic term in anatomy for various small, bulb-shaped structures or protuberances found in the body (eg the olfactory bulb, bulb of urethra). 
 But bulbs could also refer here to musical annotation, the bulbous shapes of notes on a musical score.
Split the Lark—and youll find the Music 861