Thalamus and Epithalamus

The thalamus composes about four-fifths of the diencephalon and forms most of the walls of the third ventricle (fig. 8.16). It consists of paired masses of gray matter, each positioned immediately below the lateral ventricle of its respective cerebral hemisphere. The thalamus acts primarily as a relay center through which all sensory information (except smell) passes on the way to the cerebrum. For example, the lateral geniculate nuclei relay visual information, and the medial geniculate nuclei relay auditory information, from the thalamus to the occipital and temporal lobes, respectively, of the cerebral cortex. The intralaminar nuclei of the thalamus are activated by many different sensory modalities and in turn project to many areas of the cerebral cortex. This is part of the system that promotes a state of alertness and causes arousal from sleep in response to any sufficiently strong sensory stimulus.

Corpus callosum Septum pellucidum

Genu of corpus callosum

Thalamus Anterior commissure Hypothalamus Optic chiasma

Infundibulum

Intermediate commissure

Choroid plexus of third ventricle

Corpus callosum Septum pellucidum

Corpus Callosum Pituitary

Choroid plexus of third ventricle

Splenium of corpus callosum

Pineal body

Corpora quadrigemina

Cortex of cerebellum

Pituitary gland

Mammillary body

Pons

Medulla oblongata

Splenium of corpus callosum

Pineal body

Corpora quadrigemina

Pituitary gland

Mammillary body

Pons

Cortex of cerebellum

Arbor vitae of cerebellum

Medulla oblongata

Rhombencephalon

■ Figure 8.16 A midsagittal section through the brain. (a) A diagram and (b) a photograph. Areas of the diencephalon, midbrain (mesencephalon), and hindbrain (rhombencephalon) are shaded. All of the brain outside of the these shaded areas is included in the telencephalon.

The Central Nervous System

The epithalamus is the dorsal segment of the diencephalon containing a choroid plexus over the third ventricle, where cere-brospinal fluid is formed, and the pineal gland (epiphysis). The pineal gland secretes the hormone melatonin, which may play a role in the endocrine control of reproduction (discussed in chapter 20).

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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