The Ventricular System

Is this thing really part of my brain? That's often the first thought that comes to mind when looking at images of the ventricular system. You're not quite sure whether it's an April Fool's joke or an inkblot test at the psychologist's office. But no, honestly: the inside of all our brains is partly hollow, and the cavity system actually resembles an alien wearing a helmet! And that's not all: structures with unusual shapes and unique names extend from its chambers – the Bochdalek's flower basket, for example. A quirk of nature? Of course not, because the ventricular system has various functions.

Let's take a closer look at the alien: a flattened head with eyes and an insect-like snout, an unusually long, thin, curved neck on a puny upper body with two scrawny arms and a protruding helmet on its head – this is how a cast of the ventricular system is presented in textbooks. Viewed prosier, it consists of four cavities that are connected to each other and filled with a clear fluid, the cerebrospinal fluid (liquor cerebrospinalis). This fluid not only fills the ventricles inside, but also envelops the brain and spinal cord on the outside, forming an effective buffer between the hard skull bone and the soft brain. There are no more than 150 milliliters of fluid in this cerebrospinal fluid system. Anything more than that would be unhealthy, or in other words, a hydrocephalus.

The ventricular system consists of four chambers, which can be assigned to individual sections of the brain and are numbered with Roman numerals. The two lateral ventricles, numbered I and II, are located in the two hemispheres. They correspond to the helmet of the alien but are often compared to a horn in textbooks. Both chambers are divided into the expansive anterior horn (cornu frontale) in the frontal lobe, the narrower, arched middle part (pars centralis) in the parietal lobe, the small, backward-facing posterior horn (cornu occipitale) in the occipital lobe, and the lateral, forward-running inferior horn (cornu temporale) in the temporal lobe. This borders the hippocampus at the bottom, while the other sections are bordered by the corpus callosum at the top and the thalamus at the bottom.

The third ventricle is a narrow, high, slit-shaped space. It forms the head of the alien and can be assigned to the diencephalon. It abuts the thalamus, epithalamus, and hypothalamus on the side. Where the right and left thalamus meet – at the interthalamic adhesion – there is a round recess in the third ventricle: the alien's eye. The cavity ends in four narrow, elongated bulges – two in the alien's face and two at the back of its head.

Finally, the fourth ventricle lies between the brain stem – more precisely, the medulla oblongata and pons – on the one hand and the cerebellum on the other. It forms the body of the alien with its two little arms. At their open ends, the aperturae laterales, they open onto the external cerebrospinal fluid space between the inner and middle meninges – the aforementioned “buffer” to the skull bone. Another such connection is formed by the apertura mediana – the alien's stubby tail, so to speak. The fourth ventricle continues downwards into the central canal of the spinal cord.

The four ventricles are connected to each other. Two short bulges, the foramina interventricularia, connect each lateral ventricle to the third ventricle. Whether ram's horn or alien helmet: here they are attached to the alien's head. Its long neck is actually the “water pipe of the midbrain”, the aqueductus mesencephali. Through it, the cerebrospinal fluid flows from the third to the fourth ventricle.

The unusual structures of the ventricular system develop during embryonic development after the neural tube – the origin of the central nervous system – has bulged out to form vesicles. This first creates a single cavity. However, because the different parts of the brain grow at different speeds and rates, it initially divides into three chambers, then into five. This is how the alien in the brain gradually develops.

This alien, this ventricular system, had a very special meaning for the ancient Greeks: here, according to unanimous opinion, was the seat of the human mind, even of the soul. The actual brain, on the other hand, was considered worthless by the ancient Greeks. This opinion persisted into the Middle Ages.
 

Cerebrospinal fluid is an almost cell-free and low-protein ultrafiltrate of blood, whose composition differs from all other body fluids. It is mainly produced in the venous plexuses of the ventricular system, the choroid plexuses. These highly vascularized folds of the ventricular walls look like small broccoli florets strung together. They are located in the roof of the third ventricle and extend into the middle part and lower horn of the lateral ventricle. Another filtration station is located in the fourth ventricle. Part of the broccoli protrudes through the openings in the arms into the outer cerebrospinal fluid space. With a little imagination, this resembles a flower basket and is therefore called the Bochdalek flower basket – in honor of its discoverer, the Czech anatomist Vincent Alexander Bochdalek. The ependyma, the inner single-layer cell lining of the ventricular system, controls the composition of the cerebrospinal fluid.

Our skull contains a total of about 150 milliliters of cerebrospinal fluid, 30 inside and 120 outside the brain. The outer cerebrospinal fluid space has an amazing effect: by enveloping the brain, it reduces its effective weight on the bones from 1500 grams to just 50 grams.

Every day, the body produces about half a liter of cerebrospinal fluid, meaning that the entire contents are replaced about three times a day. If more is produced than is absorbed, the cerebrospinal fluid accumulates in the skull, leading to external hydrocephalus. In infants, the still flexible skull begins to enlarge. However, the narrow aqueduct between the third and fourth ventricles can also become blocked, causing only the ventricular system I-III to expand – resulting in what is known as internal hydrocephalus. In either case, the result is severe headaches and potential brain dysfunction.

Computed tomography and magnetic resonance imaging can be used to easily visualize the ventricular system. The physician can then use cross-sectional images to assess whether it is larger than normal. In the case of hydrocephalus, the physician will attempt to restore normal cerebrospinal fluid drainage using a drain.
 

First published on August 8, 2011
Last updated on August 5, 2025