An ear is the organ that enables hearing and, in mammals, body balance using the vestibular system. In mammals, the ear is usually described as having three parts—the outer ear, the middle ear and the inner ear. The outer ear consists of the pinna and the ear canal. Since the outer ear is the only visible portion of the ear in most animals, the word "ear" often refers to the external part alone. The middle ear includes the tympanic cavity and the three ossicles. The inner ear sits in the bony labyrinth, and contains structures which are key to several senses: the semicircular canals, which enable balance and eye tracking when moving; the utricle and saccule, which enable balance when stationary; and the cochlea, which enables hearing. The ears of vertebrates are placed somewhat symmetrically on either side of the head, an arrangement that aids sound localisation. The ear develops from the first pharyngeal pouch and six small swellings that develop in the early embryo called otic placodes, which are derived from ectoderm. The ear may be affected by disease, including infection and traumatic damage. Diseases of the ear may lead to hearing loss, tinnitus and balance disorders such as vertigo, although many of these conditions may also be affected by damage to the brain or neural pathways leading from the ear. The ear has been adorned by earrings and other jewelry in numerous cultures for thousands of years, and has been subjected to surgical and cosmetic alterations.
In human anatomy, the carotid sinus is a dilated area at the base of the internal carotid artery just superior to the bifurcation of the internal carotid and external carotid at the level of the superior border of thyroid cartilage. The carotid sinus extends from the bifurcation to the "true" internal carotid artery. The carotid sinus is sensitive to pressure changes in the arterial blood at this level. It is the major baroreception site in humans and most mammals. Structure
The carotid sinus is the reflex area of the carotid artery, consisting of baroreceptors which monitor blood pressure. Function
The carotid sinus contains numerous baroreceptors which function as a "sampling area" for many homeostatic mechanisms for maintaining blood pressure. The carotid sinus baroreceptors are innervated by the carotid sinus nerve, which is a branch of the glossopharyngeal nerve (CN IX). The neurons which innervate the carotid sinus centrally project to the solitary nucleus in the medulla of the brainstem. The solitary nucleus indirectly modulates the activity of sympathetic and parasympathetic neurons in the medulla and pons through the hypothalamus. These neurons then regulate the autonomic control of the heart and blood vessels. The aortic arch baroreceptors are innervated by the aortic nerve (nerve of Cyon, Ludwig nerve), which combines with CN X (vagus nerve) and travels to the NTS. Clinical significance
It is a sensitive site of the body because stimulation can drive large-scale reflex effects throughout the body. This can be used therapeutically in treatment of resistant hypertension by baroreflex activation.