The orbit of the eye is formed by the following bones: maxillae, lacrimals, ethmoid, zygomatics, sphenoid, and frontal. The eye itself fits into the recess formed by these bones (the orbit). Protecting the anterior aspects of the eye are the eyelids (or palpebrae). Covering the posterior surface of the eyelids and covering the anterior portion of the sclera is a thin membrane called the conjunctiva. It is continuous with the epithelium of the cornea. Located in the superior, anterolateral aspect of the orbit of each eye is a lacrimal gland. The lacrimal gland is divided into two parts: the superior lacrimal gland and the inferior lacrimal gland. Leading from these glands are six to twelve lacrimal ducts that empty tears (lacrimal fluid) onto the surface of the conjunctiva. The tears bathe the surface of the eye and then pass medially into two openings called lacrimal punctae. These two openings lead into the superior lacrimal duct (located in the upper eyelid) and the inferior lacrimal duct (located in the lower eyelid). These two ducts open into a large cavity called the lacrimal sac. The lacrimal sac empties into the nasolacrimal duct. This duct opens into the nasal cavity. The moisture from the tears then serves to humidify the air we breath. Located between the lacrimal punctae is a mound of tissue that contains sebaceous and sweat glands called the caruncle. The caruncle secretes a whitish material that collects in the corner of each eye. In the medial corner of the eye is a small fold called the plica semilunaris. It is believed by some to be a homologue (similarity in structure and origin of two parts in different species of animals) of the nictitating membrane, which is a third eyelid present in lower vertebrate animals.
Figure 24.1 Nasolacrimal Apparatus
Inferior lacrimal duct Inferior lacrimal gland Iris Lacrimal duct Lacrimal punctum Lacrimal sac Lens Nasolacrimal duct Plica semilunaris Posterior chamber Pupil Sclera Superior lacrimal duct Superior lacrimal gland Suspensory ligament
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.