The human skull is a complex bony structure that serves to protect the brain and form the framework of the face. It is made up of numerous distinct bones that are typically organized into two major categories based on their function and location. The lacrimal bone, one of the smallest and most fragile bones in the entire skull, is a paired structure. Its name is derived from the Latin word lacrima, meaning “tear,” hinting at its specialized anatomical role.
Defining Facial and Cranial Bones
The 22 bones of the skull are primarily divided into the neurocranium and the viscerocranium, which are more commonly known as the cranial and facial bones, respectively. The neurocranium is the protective vault that surrounds the brain, acting as the skull’s roof and base. Bones in this category include the frontal, parietal, temporal, and occipital bones, designed to safeguard the central nervous system and house the inner ear structures.
The viscerocranium, or facial skeleton, forms the anterior and inferior parts of the skull, providing the underlying structure for the face. These bones are instrumental in forming the orbital cavities for the eyes, the nasal cavity, and the oral cavity, while also supporting the teeth. Examples of facial bones are the maxilla, mandible, and zygomatic bones. This group of bones supports the soft tissues of the face and creates passageways for breathing and eating.
Location and Classification as a Facial Bone
The lacrimal bone is classified as a facial bone, belonging to the viscerocranium. This small, thin bone is situated in the anterior portion of the medial wall of the orbit, the bony socket housing the eye. Its delicate, generally rectangular shape is comparable in size to a fingernail, making it the smallest paired bone of the face.
It is positioned between the frontal process of the maxilla anteriorly and the orbital plate of the ethmoid bone posteriorly. The lacrimal bone’s classification is based on its primary function of contributing to the facial framework, specifically the orbit and the nasal cavity, rather than the braincase. The bone articulates with four other bones: the frontal bone superiorly, the ethmoid bone posteriorly, the maxilla anteriorly and inferiorly, and the inferior nasal concha inferiorly.
The Role in the Lacrimal Apparatus
The lacrimal bone’s physical structure is specifically adapted to support the tear drainage system, known as the lacrimal apparatus. A prominent feature on the bone’s orbital surface is a vertical ridge called the posterior lacrimal crest. This crest defines a vertical groove, the lacrimal sulcus, on the anterior side of the bone.
The lacrimal sulcus joins the adjacent frontal process of the maxilla to form the lacrimal fossa. This fossa houses the lacrimal sac, which collects tears from the eye’s surface. Below the lacrimal sac, the lacrimal bone contributes a descending process that helps form the nasolacrimal canal. This canal provides the passage for the nasolacrimal duct, which drains tears into the inferior meatus of the nasal cavity.