Visible Human Transverse Section Through the Head

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Module Name: a_vm1110
Module Title: Visible Human Transverse Section Through the Head
Image Info: Gross specimen, transverse section
Created by: Lynn Bry
Date: Feb 4th, 1997


Key points: eye | lens | medial rectus | lateral rectus | ethmoid sinus | sphenoid sinus | nasal septum | temporalis muscle | brain |

Introduction: This transverse section cuts through the head at the level of the eye. The lenses of the eye can be seen at the top, just above the bluish vitreous humor of the eyeball. The lateral and medial rectus muscle lie on either side of the eye. The ethmoid air sinuses within the nose, as well as the sphenoid air sinuses, and the nasal septum lie between the orbits.

At this level the temporal and occipital lobes of the cerebrum can be clearly seen. A small region of the cerebellum lies in the middle of the space occupied by the cerebrum.


brain: This section primarily cuts through the temporal and optic lobes of the cerebrum. Different regions of the cerebrum carry out specific tasks regarding thought, processing of sensory input, and other cognitive functions. The temporal lobe is involved with smell and language, and certain behaviours such as "self-preservation," hostility and sexual behaviors. Parts of the temporal lobe are also involved in processing input from the auditory nerve. The occipital lobe primarily carries out the processing of visual information.

cerebellum: The cerebellum lies below the largest part of the brain, the cerebrum. Neurons in the cerebellum help control motor coordination, including control over muscle tone, equilibrium of the body, and a positional sense of where the body is in space. Walking in a straight line by placing your feet heel to toe requires input from the cerebellum.

cranium: More than 25 individual bones form the cranium (skull). The bones come together at irregular suture lines, fitting together like pieces of a three-dimensional puzzle.

ethmoid_sinus: The ethmoid air sinuses lie behind the nasal passageways. These air cells are covered with an epithelium. Epithelial cells secrete mucous and help to warm and moisten air destined for the lungs.

eye: This section through the eyeball demonstrates the lens, located at the front of the eyeball, and the bluish vitreous humor, a thick gelatinous material which fills the interior of the eye.

The area of the eye in front of the lens is divded into the anterior and posterior chambers. The anterior chamber occupies the space between the cornea and iris muscle, while the posterior chamber represents the space from the iris to the lens. The ciliary body in the posterior chamber secretes the aqueous humor, a fluid of thinner consistency than the vitreous humor. Aqueous humor circulates throughout the anterior chamber and is aborbed through a small encircling canal in the anterior chamber (Canal of Schlemm). Improper drainage of aqueous humor leads to a buildup of pressure in the anterior of the eye, a serious condition known as glaucoma.

internal_carotid_artery: The internal carotid arteries branch off the common carotid arteries in the neck just below the mandible. They give off no branches until meeting with the communicating branches of the basilar artery. The basilar artery is formed by the two vertebral arteries which arrive at the posterior of the brain via the vertebral column.

Together the basilar and internal carotid arteries form the Circle of Willis, the primary vasular structure supplying blood to the brain. Blood from the internal carotids primarily goes to the frontal and temporal lobes.

lateral_rectus: Contraction of the lateral rectus muscle of the eye moves the eye laterally, allowing vision to the side (abduction of the eye).

The abducens nerve (CN VI, or 6th cranial nerve) innervates this muscle. Aside from the superior oblique muscle of the eye (supplied by CN IV, trochlear), all other eye muscles are innervated by the oculomotor nerve (CN III). These muscles include the medial rectus muscles, shown in this section.

lens: The lens of the eye is made of a translucent, proteinaceous material. A suspensory ligament on either side of the lens attaches it to the ciliary muscles, 2 per eye. These muscles lie just beyond the pupillary constrictor muscle (the iris), in the same plane as the suspensory ligament. When stimulated, the muscle contracts, releasing tension on the suspensory ligament. The decrease in tension allows the lens material to form a more spherical shape, i.e. fatten in the middle.

The entire mechanism is called the accomodation reflex. It allows for near vision. The distortion fo the lens alters the focusing of light on the back of the eye, the retina.

medial_rectus: Contraction of the medial rectus muscle helps moves the eye to the middle, as when crossing one's eyes (adduction of the eye).

The oculomotor nerve (CN III, or cranial nerve #3) innervates this muscle. In addition, CN III controls all eye muscles except the lateral rectus (also shown in this section) which is controlled by CN VI (abducens nerve), and the superior oblique muscle (not shown) which is controlled by CN IV (trochlear).

optic_chiasm: A small area of the optic chiam appears in this section. The chiasm represents the "crossing-over" of nerve fibers from the right and left optic nerves of the eye. If you were to divide each eye in half along the sagittal plane, fibers from the outside of the right eye continue to the right side of the brain, ultimately to the R. occipital lobe. Fibers from the left/inside region of the right eye cross at the chiasm and continue to the left side of the brain. Likewise, the inside region of the left eye crosses to the right side of the brain, while fibers from the outside area of the eye continue to the left hemisphere.

orbital_fat: The posterior region of the eye socket (orbit) contains fatty tissue. This tissue helps cushion the eye within the orbit.

sphenoid_bone: The spenoid bone forms much of the interior structure of the cranium. It houses the sphenoid air sinuses, as well as the pituitary gland and hypothalamus. These important endocrine organs reside in a bony encasement called the sella tursica or turkish saddle, so named for its 3D resemblance to a saddle. Both secrete factors that regulate endocrine organs such as the thyroid and adrenal glands.

sphenoid_sinus: The sphenoid sinus resides within the sphenoid bone of the cranium. This sinus connects with the nasal passages, and is part of an intricate system of air passageways that warm incoming air. The hollow sinsuses also make the cranium lighter.

temporalis_muscle: The temporalis muscle runs along the sides of the cranium, starting in the region of your "temples." The muscle is one of mastication (chewing) as it inserts on the jaw bone (mandible). Contraction of the temporalis helps 'close' the jaw while chewing. The other primary muscle of mastication, the masseter can be felt clenching the teeth.

zygomatic_bone: The zygomatic bone forms the cheeks.

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