Dynamic Equilibrium

Each semicircular canal follows a circular path about 6 millimeters in diameter. The three bony semicircular canals lie at right angles to each other and occupy three different planes in space. Two of them, the superior canal and the posterior canal, stand vertically, whereas the third, the lateral canal, is horizontal. Their orientations closely approximate the three body planes (see chapter 1, p. 22).

Suspended in the perilymph of each bony canal is a membranous semicircular canal that ends in a swelling called an ampulla (am-pul'lah). The ampullae communicate with the utricle of the vestibule.

An ampulla contains a septum that crosses the tube and houses a sensory organ. Each of these organs, called a crista ampullaris, has a number of sensory hair cells and supporting cells. As in the maculae, the hairs of the hair cells extend upward into a dome-shaped gelatinous mass called the cupula. Also, the hair cells are connected at their bases to nerve fibers that make up part of the vestibular branch of the vestibulocochlear nerve (fig. 12.21).

Rapid turns of the head or body stimulate the hair cells of the crista. At such times, the semicircular canals move with the head or body, but the fluid inside the membranous canals tends to remain stationary because of inertia. This bends the cupula in one or more of the canals in a direction opposite that of the head or body

Essentials of Human Physiology

Essentials of Human Physiology

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.

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