Special Senses

Eye

The human eye is a complex sensory organ responsible for vision. It consists of several anatomical structures that work together to focus light and transmit visual information to the brain.

• Lacrimal gland: Produces tears to lubricate and protect the eye.

• Anterior cavity of eye: The space in front of the lens, filled with aqueous humor.

• Posterior cavity of eye: The space behind the lens, filled with vitreous humor.

• Cornea: Transparent, dome-shaped surface that covers the front of the eye and refracts light.

• Sclera: The white, fibrous outer layer that maintains the shape of the eye.

• Pupil: The opening in the center of the iris that regulates the amount of light entering the eye.

• Iris: Colored part of the eye that controls pupil size.

• Lens: Transparent structure that focuses light onto the retina.

• Ciliary body: Contains muscles that adjust lens shape for focusing.

• Retina: Light-sensitive layer containing photoreceptors (rods and cones).

• Fovea centralis: Area of the retina with the highest concentration of cones, responsible for sharp central vision.

• Optic disc: Region where the optic nerve exits the eye; also known as the "blind spot" due to lack of photoreceptors.

Example: The lens changes shape (accommodation) to focus on near or distant objects.

Ear

The ear is the organ of hearing and balance, divided into external, middle, and inner regions. Each part contains specialized structures for sound transmission and equilibrium.

• Auricle (pinna): External part that collects sound waves.

• External acoustic meatus: Canal that transmits sound to the eardrum.

• Tympanic membrane: Eardrum; vibrates in response to sound.

• Malleus, incus, stapes: Auditory ossicles that transmit vibrations from the eardrum to the inner ear.

• Tensor tympani muscle: Dampens loud sounds by tensing the tympanic membrane.

• Oval window: Membrane-covered opening to the inner ear.

• Round window: Allows movement of fluid within the cochlea.

• Cochlea: Spiral-shaped organ for hearing; contains hair cells that convert sound to nerve impulses.

• Semicircular canals: Three looped structures involved in balance and spatial orientation.

Example: Vibrations from the stapes at the oval window initiate fluid movement in the cochlea, leading to hearing.

Endocrine System

Major Glands and Structures

The endocrine system consists of glands that secrete hormones directly into the bloodstream, regulating various physiological processes.

• Hypothalamus: Brain region controlling pituitary gland and homeostasis.

• Pituitary gland: Master gland with anterior (pars distalis, pars tuberalis, pars intermedia) and posterior (pars nervosa) lobes.

• Pinealocytes: Cells of the pineal gland, produce melatonin.

• Thyroid gland: Contains follicular cells (produce thyroid hormones) and colloid (stores hormones).

• Parathyroid gland: Chief and oxyphil cells regulate calcium levels.

• Thymic corpuscles: Structures in the thymus involved in T-cell maturation.

• Pancreas: Contains acinar cells (exocrine), islets of Langerhans (endocrine), and ducts.

• Adrenal gland: Cortex (zona glomerulosa, fasciculata, reticularis) and medulla; produces corticosteroids and catecholamines.

• Testicular and ovarian follicular cells: Produce sex hormones.

Example: The hypothalamus releases hormones that stimulate the pituitary to secrete growth hormone.

Blood

Components and Tests

Blood is a connective tissue composed of plasma and formed elements, essential for transport, immunity, and homeostasis.

• Plasma: Liquid matrix containing water, proteins, nutrients, and waste.

• Erythrocyte: Red blood cell; transports oxygen via hemoglobin.

• Leukocytes: White blood cells; include neutrophils, eosinophils, basophils, lymphocytes, and monocytes.

• Thrombocyte: Platelet; involved in clotting.

• Fibrin: Protein that forms the mesh of a blood clot.

• Blood type tests: Identify antigens (A, B, Rh) on erythrocytes.

Example: Type A blood has A antigens on erythrocytes and anti-B antibodies in plasma.

Additional info: Blood typing is crucial for safe transfusions.

Heart

Anatomical Structures

The heart is a muscular organ that pumps blood throughout the body. It consists of chambers, valves, and associated vessels, each with specific functions.

• Chambers: Right and left atria, right and left ventricles.

• Valves: Bicuspid (mitral), tricuspid, pulmonary semilunar, aortic semilunar.

• Major vessels: Aorta, pulmonary trunk, superior/inferior vena cava, coronary arteries and veins.

• Other structures: Myocardium (muscle layer), endocardium (inner lining), epicardium (outer layer), fossa ovalis, chordae tendineae, papillary muscles.

Example: The left ventricle has the thickest myocardium to generate high pressure for systemic circulation.

Additional info: The heart's conduction system (not listed) includes the SA node, AV node, bundle of His, and Purkinje fibers.