The third ventricle is one of the four connected cerebral ventricles of the ventricular system within the mammalian brain. It is a slit-like cavity formed in the diencephalon between the two thalami, in the midline between the right and left lateral ventricles, and is filled with cerebrospinal fluid (CSF).
Running through the third ventricle is the interthalamic adhesion, which contains thalamic neurons and fibers that may connect the two thalami.
The fibers of the oculomotor nerve arise from a nucleus in the midbrain, which lies in the gray substance of the floor of the cerebral aqueduct and extends in front of the aqueduct for a short distance into the floor of the third ventricle. From this nucleus the fibers pass forward through the tegmentum, the red nucleus, and the medial part of the substantia nigra, forming a series of curves with a lateral convexity, and emerge from the oculomotor sulcus on the medial side of the cerebral peduncle.
The nucleus of the oculomotor nerve does not consist of a continuous column of cells, but is broken up into a number of smaller nuclei, which are arranged in two groups, anterior and posterior. Those of the posterior group are six in number, five of which are symmetrical on the two sides of the middle line, while the sixth is centrally placed and is common to the nerves of both sides. The anterior group consists of two nuclei, an antero-medial and an antero-lateral.
In the human brain, the diencephalon (or interbrain) is a division of the forebrain (embryonic prosencephalon). It is situated between the telencephalon and the midbrain (embryonic mesencephalon). The diencephalon has also been known as the tweenbrain in older literature. It consists of structures that are on either side of the third ventricle, including the thalamus, the hypothalamus, the epithalamus and the subthalamus.
The diencephalon is one of the main vesicles of the brain formed during embryonic development. During the third week of development a neural tube is created from the ectoderm, one of the three primary germ layers, and forms three main vesicles: the prosencephalon, the mesencephalon and the rhombencephalon. The prosencephalon gradually divides into the telencephalon (the cerebrum) and the diencephalon.
The thalamus (pl.: thalami; from Greek θάλαμος, "chamber") is a large mass of gray matter on the lateral wall of the third ventricle forming the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, known as the thalamocortical radiations, allowing hub-like exchanges of information. It has several functions, such as the relaying of sensory and motor signals to the cerebral cortex and the regulation of consciousness, sleep, and alertness.
Anatomically, the thalami are paramedian symmetrical structures (left and right), within the vertebrate brain, situated between the cerebral cortex and the midbrain. It forms during embryonic development as the main product of the diencephalon, as first recognized by the Swiss embryologist and anatomist Wilhelm His Sr. in 1893.
Circumventricular organs (CVOs) (circum-: around ; ventricular: of ventricle) are structures in the brain characterized by their extensive and highly permeable capillaries, unlike those in the rest of the brain where there exists a blood–brain barrier (BBB) at the capillary level. Although the term "circumventricular organs" was originally proposed in 1958 by Austrian anatomist Helmut O. Hofer concerning structures around the brain ventricular system, the penetration of blood-borne dyes into small specific CVO regions was discovered in the early 20th century. The permeable CVOs enabling rapid neurohumoral exchange include the subfornical organ (SFO), the area postrema (AP), the vascular organ of lamina terminalis (VOLT — also known as the organum vasculosum of the lamina terminalis (OVLT)), the median eminence, the pituitary neural lobe, and the pineal gland.
The circumventricular organs are midline structures around the third and fourth ventricles that are in contact with blood and cerebrospinal fluid, and they facilitate special types of communication between the central nervous system and peripheral blood. Additionally, they are an integral part of neuroendocrine function. Highly permeable capillaries allow the CVOs to act as an alternative route for peptides and hormones in the neural tissue to sample from and secrete to circulating blood. CVOs also have roles in body fluid regulation, cardiovascular functions, immune responses, thirst, feeding behavior and reproductive behavior.
The lateral ventricles are the two largest ventricles of the brain and contain cerebrospinal fluid. Each cerebral hemisphere contains a lateral ventricle, known as the left or right lateral ventricle, respectively.
Each lateral ventricle resembles a C-shaped cavity that begins at an inferior horn in the temporal lobe, travels through a body in the parietal lobe and frontal lobe, and ultimately terminates at the interventricular foramina where each lateral ventricle connects to the single, central third ventricle. Along the path, a posterior horn extends backward into the occipital lobe, and an anterior horn extends farther into the frontal lobe.
The fourth ventricle is one of the four connected fluid-filled cavities within the human brain. These cavities, known collectively as the ventricular system, consist of the left and right lateral ventricles, the third ventricle, and the fourth ventricle. The fourth ventricle extends from the cerebral aqueduct (aqueduct of Sylvius) to the obex, and is filled with cerebrospinal fluid (CSF).
The fourth ventricle has a characteristic diamond shape in cross-sections of the human brain. It is located within the pons or in the upper part of the medulla oblongata. CSF entering the fourth ventricle through the cerebral aqueduct can exit to the subarachnoid space of the spinal cord through two lateral apertures and a single, midline median aperture.
The cerebral aqueduct (aqueduct of the midbrain, aqueduct of Sylvius, Sylvian aqueduct, mesencephalic duct) is a small, narrow tube connecting the third and fourth ventricles of the brain. The cerebral aqueduct is a midline structure that passes through the midbrain. It extends rostrocaudally through the entirety of the more posterior part of the midbrain. It is surrounded by the periaqueductal gray (central gray), a layer of gray matter.
Congenital stenosis of the cerebral aqueduct is a cause of congenital hydrocephalus.
The interthalamic adhesion (also known as the massa intermedia, intermediate mass or middle commissure) is a flattened band of tissue that connects both parts of the thalamus at their medial surfaces. The medial surfaces form the upper part of the lateral wall to the third ventricle.
In humans, it is only about one centimeter long – though in females, it is about 50% larger on average. Sometimes, it is in two parts – and 20% of the time, it is absent. In other mammals, it is larger.