Sebaceous gland the hair shaft. The structure consisting of hair, hair follicle, arrector pili muscles, and sebaceous gland is an epidermal invagination known as a "pilosebaceous unit". Sebaceous glands are also found in hairless areas (glabrous skin) of the eyelids, nose, penis, labia minora, the inner mucosal membrane of the cheek, and nipples. Some sebaceous glands have unique names. Sebaceous glands on the lip and mucosa of the cheek, and on the genitalia, are known as Fordyce spots, and glands on the eyelids are known as meibomian glands. Sebaceous glands of the breast are also known as Montgomery's glands. Sebaceous glands are
Sealant performing these functions. A sealant may be viscous material that has little or no flow characteristics and which stay where they are applied; or they can be thin and runny so as to allow it to penetrate the substrate by means of capillary action. Anaerobic acrylic sealants (generally referred to as impregnants) are the most desirable, as they are required to cure in the absence of air, unlike surface sealants that require air as part of the cure mechanism that changes state to become solid, once applied, and is used to prevent the penetration of air, gas, noise, dust, fire,