Completing the vascular layer of the eyeball anteriorly is the iris (Fig. 8.108). This circular structure, projecting outward from the ciliary body, is the colored part of the eye with a central opening (the pupil). Controlling the size of the pupil are smooth muscle fibers (sphincter pupillae) and myoepithelial cells (dilator pupillae) within the iris (Fig. 8.110):
Fibers arranged in a circular pattern make up the sphincter pupillae muscle (Table 8.9), which is innervated by parasympathetics—contraction of its fibers decreases or constricts the pupillary opening.
Contractile fibers arranged in a radial pattern make up the dilator pupillae muscle, which is innervated by sympathetics—contraction of its fibers increases or dilates the pupillary opening.
Inner layer of the eyeball
The inner layer of the eyeball is the retina (Fig. 8.108). It consists of two parts. Posteriorly and laterally is the optic part of the retina, which is sensitive to light, and anteriorly is the nonvisual part, which covers the internal surface of the ciliary body and the iris. The junction between these parts is an irregular line (the ora serrata).
Optic part of the retina
The optic part of the retina consists of two layers, an outer pigmented layer and an inner neural layer:
The pigmented layer is firmly attached to the choroid and continues anteriorly over the internal surface of the ciliary body and iris.
The neural layer, which can be further subdivided into its various neural components, is only attached to the pigmented layer around the optic nerve and at the ora serrata.
It is the neural layer that separates in the case of a detached retina.
Several obvious features are visible on the posterior surface of the optic part of the retina.
The optic disc is where the optic nerve leaves the retina (Fig. 8.109). It is lighter than the surrounding retina and branches of the central retinal artery spread from this point outward to supply the retina. As there are no lightsensitive receptor cells in the optic disc, it is referred to as a blind spot in the retina.
Lateral to the optic disc a small area with a hint of yellowish coloration is the macula lutea with its central depression, the fovea centralis (Fig. 8.109). This is the thinnest area of the retina and visual sensitivity here is higher than elsewhere in the retina because it has fewer rods (light-sensitive receptor cells that function in dim light and are insensitive to color) and more cones (light-sensitive receptor cells that respond to bright light and are sensitive to color).
The ear is the organ of hearing and balance. It has three parts (Fig. 8.113):
The first part is the external ear consisting of the part attached to the lateral aspect of the head and the canal leading inward.
The second part is the middle ear—a cavity in the petrous part of the temporal bone bounded laterally, and separated from the external canal, by a membrane and connected internally to the pharynx by a narrow tube.
The third part is the internal ear consisting of a series of cavities within the petrous part of the temporal bone between the middle ear laterally and the internal acoustic meatus medially.
The internal ear converts the mechanical signals received from the middle ear, which start as sound captured by the external ear, into electrical signals to transfer information to the brain. The internal ear also contains receptors that detect motion and position.
The external ear consists of two parts. The part projecting from the side of the head is the auricle (pinna) and the canal leading inward is the external acoustic meatus.
The auricle is on the side of the head and assists in capturing sound. It consists of cartilage covered with skin and arranged in a pattern of various elevations and depressions (Fig. 8.114).
The large outside rim of the auricle is the helix. It ends inferiorly at the fleshy lobule, the only part of the auricle not supported by cartilage.