Lateral Ventricles (Pl. 21)

The lateral ventricles (1st and 2nd) are each divided into a number of parts: anterior horn, body, atrium, posterior horn, and inferior horn.

Anterior or Frontal Horn

The part of the lateral ventricle anterior to the interventricular foramen (of Monro) is called the anterior or frontal horn(Pls. 31, 32; MRIs 1, 2, 8, 9, 10, 16). Medially, it is bounded by the septum pellucidum, fornix, and genu of the corpus callosum. Laterally, the head of the caudate bulges into the frontal horn while its floor is composed of the rostrum of the corpus callosum.


The body of the lateral ventricle (Pls. 26, 27, 28, 29 MRIs 4, 5, 6, 16, 17) extends from the foramen of Monro to the splenium of the corpus callosum. Like the anterior horn, the medial border of the body continues to be the septum pellucidum and its roof remains bounded by the corpus callosum. Laterally, the body is adjacent to the caudate nucleus and its floor is formed by the thalamus, fornix, and choroid plexus.

Atrium or Trigone

The trigone (or atrium) (Pls. 30, 31, 32, MRIs 7, 8, 9, 10) of the lateral ventricle is its most expanded section, and it is triangular in shape. Anteriorly, it is related to the fornix and pulvinar. It contains an abundant tuft of choroid plexus, the glomus, along its anterior wall (Pls 30, 31, 32; MRIs 9, 10, 18).

Inferior or Temporal Horn

The inferior or temporal horn (Pls. 26, 27, 28, 29, MRIs 3, 4, 6, 11) is within the temporal lobe. It ends about 3 cm behind the temporal pole. Its roof is formed by the tapetum of the corpus callosum. Medially, it is bounded by the tail of the caudate nucleus and the hippocampus, and it contains choroid plexus in its superior-medial aspect.

Posterior or Occipital Horn

The posterior or occipital horn (Pl. 31) is the most variable part of the ventricular system. Medially, the calcar avis, formed by the calcarine fissure, bulges into the occipital horn.

Interventricular Foramen (of Monro)

The foramen of Monro (Pls. 5, 12, 13, MRI 14) is the passageway between each lateral ventricle and the single third ventricle, located in the median plane. Note its relationships the column of the fornix anteriorly and the anterior tubercle of the thalamus posteriorly.

Third Ventricle

The third ventricle is bordered laterally by the thalamus above the hypothalamic sulcus,and by the hypothalamus below (Pls. 12, 13 MRIs 3, 4, 5, 9, 10, 11, 12). Sometimes a connection between the thalami, the interthalamic adhesion or mass intermedia, bridges across the middle of the third ventricle. The anterior wall of the 3rd ventricle is the lamina terminalis. Its roof is formed by the tela choroidea which contains the internal cerebral veins and choroid plexus.

Cerebral Aqueduct (of Sylvius)

The cerebral aqueduct or iter is located in the midbrain and connects the third and fourth ventricles (Pls. 12, 13, MRIs 12, 13, 14).

Fourth Ventricle

The fourth ventricle is a single cavity whose rhomboid shaped floor is in the roof of the pons and rostral medulla. (Pl. 13; MRIs 7, 12, 13, 14). It expands posteriorly in an inverted kite-shape, with its roof bounded by the superior and inferior medullary vela and the superior cerebellar peduncles. Choroid plexus is attached to the inferior medullary velum and extends laterally through the lateral apertures (foramina of Luschka) into the subarachnoid space at the cerebellar angle. The lateral borders of the fourth ventricle are the three cerebellar peduncles. The median aperture (foramen of Magendie) empties into the vallecula, a midline extension of the cisterna magnum.

Cerebrospinal Fluid Flow and Hydrocephalus

Be able to trace the flow of cerebrospinal fluid from the choroid plexus in the inferior horns of the lateral ventricles to the arachnoid villi in the superior sagittal sinus, where it is chiefly absorbed. Blockage of the flow of cerebrospinal fluid within the ventricular system results in obstructive hydrocephalus, whereas blockage in the subarachnoid space results in communicating hydrocephalous.