Final answer:
The substance expelled from a compound alveolar gland is milk. The mammary glands produce breast milk, which is secreted into lactiferous ducts.
Step-by-step explanation:
The substance expelled from a compound alveolar gland would be milk. The mammary glands, which are modified sweat glands, produce breast milk. The milk is secreted into lactiferous ducts that open on the surface of the nipple.