A Lateral Ventricle Curves through Each Cerebral Hemisphere

Each lateral ventricle is basically a C-shaped structure. This C shape curves from an inferior horn in the temporal lobe through a body in the parietal lobe and a bit of the frontal lobe, ending at the interventricular foramen where each lateral ventricle joins the third ventricle. Along this C-shaped course two extensions emerge—a posterior horn that extends backward into the occipital lobe and an anterior horn that extends farther into the frontal lobe (Fig. 5-1; see THB6 Figure 5-2, p. 101). The expanded area where the body and the inferior and posterior horns meet is called the atrium. Each lateral ventricle represents the cavity of an embryonic telencephalic vesicle, so telencephalic structures like the caudate nucleus and the hippocampus border much of it; the thalamus, a diencephalic derivative, also forms part of its floor (THB6 Figures 3-19 to 3-24, pp. 69-71).

Figure 5-1 The ventricles. 3, Third ventricle; 4, fourth ventricle; Aq, aqueduct; IA, interthalamic adhesion; IF, interventricular foramen.

(Thanks to Dr. John Sundsten.)

The Third Ventricle Is a Midline Cavity in the Diencephalon

The third ventricle is a midline slit in the diencephalon, with walls formed by the hypothalamus and much of the thalamus. The thalamus grows right across the third ventricle in most human brains, forming an interthalamic adhesion, so that the ventricle winds up looking like a misshapen doughnut.

The Fourth Ventricle Communicates with Subarachnoid Cisterns

The fourth ventricle extends from the cerebral aqueduct of the midbrain, through which it communicates with the third ventricle, to a mid-medullary level, where it narrows down into the vestigial central canal of the caudal medulla and the spinal cord. It reaches its widest extent at the pontomedullary junction, where it extends into a lateral recess on each side. The tentlike roof of the fourth ventricle pokes up into the cerebellum.

The ventricular system communicates with subarachnoid space through three apertures of the fourth ventricle, a lateral aperture at the end of each lateral recess and a median aperture above the point where the fourth ventricle narrows down into the central canal.

The Ventricles Contain Only a Fraction of the CSF

Although CSF is made in the ventricles, most of it is physically located in subarachnoid space (about 25 mL in the ventricles vs. 150 mL or so in subarachnoid space).