Central nervous system disorders

I./1.3.: Third ventricle

I./1.3.1.: General characteristics

A single midline cavity, often appearing as a narrow gap between the two thalamuses. Embryologically its cavity and walls mostly belong to the diencephalon, only the anterior part originates from the telecephalon (telencephalon impar).

I./1.3.2.: Inferior wall

The bottom of the third ventricle is formed by the hypothalamus. If examined with endoscope in vivo or in unfixed cadavers, it can be well seen that the two corpus mamillare and behind them the top of the arteria basilaris shows through. In forward direction the infundibulum hypophysealis appears with the ventricle’s lateral deflection called recessus infundibuli. In front of this the chiasma opticum bulges in, shaped like a strong, crosswise beam. Between the chiasma and the lamina terminalis, the recessus opticus can be found.

I./1.3.3.: Lateral walls

The lateral walls of the ventricle are formed by the two thalamuses; the adhesio interthalamica (massa intermedia) bridges the gap between them. A shallow sulcus hypothalamicus runs along between the thalamuses and the hypothalamus. On the upper part of the thalamuses, on the border of their dorsal and lateral surface a brain tissue stripe can be seen, the stria medullaris thalami.

I./1.3.4.: Anterior wall

The thin lamina terminalis forms the front wall. On each side of its inner surface the columna fornicis extends downwards. The parts visible here are called the pars libera columnae fornicis, while the part that disappears into the hypothalamus is called pars tecta columnae fornicis. Horizontally between the two columnas a short section of the comissura anterior can be found. These three structures resemble the letter A, the depression between the structures is called recessus triangularis.

I./1.3.5.: Posterior wall

On the upper part of the posterior wall the two stria medullaris thalami is connected, forming the comissura habenularum, from which the corpus pineale hangs down towards the cisterna quadrigemina. Above this the recessus suprapinealis, under this the recessus pinealis and the comissura posterior is located. A tight junction starts where the posterior and inferior wall meets, this is the az aquaeductus cerebri (Sylvii), which leads to the fourth ventricle.

I./1.3.6.: Superior wall

In the stronger sense, the roof of the ventricle is formed by a brain tissue lamina, the lamina choroidea epithelialis ventriculi tertii, which is a remaining, one cell thick wall section of the diencephalon vesicle. In a broader sense the roof consists of the velum interpositum and the above mentioned lamina. The velum interdepisotum is a cistern in the subarachnoid space, which is formed by the pia mater’s deep enfolding under the splenium corporis callosi and above the crura et comissura fornicis. Similar to the other cisterns, CSF can be found in this enfolding pia sack. In older descriptions this area is sometimes referred to as cavum psalterii. The name originates from the fact that the comissura fornicis also used to be called as psalterium or lyra Davidis.