 |
VI./1.2.: The brain’s commissural tracts
Their task is the synchronization of the two hemisphere’s function. As commissural tracts correspond to fibre bundles connecting identical areas of the two hemispheres, their location can be mainly interpreted in a frontal plane. Thus we can make them visible by cutting brains preserved in formaldehyde in half perpendicular to this, namely in the mediansagittal plane.
Largest is the corpus callosum. On a brain cut in half one can distinguish its anterior, beaklike rostrum, the rostrum’s thin, roughly horizontal lamina rostralis which continues into the vertical lamina terminalis. Backward from the rostrum the corpus callosum continues in the „knee” (genu), then the main part, the corpus seu truncus corporis callosi runs backwards, and after a turn downward and to the front, it ends in the splenium corporis callosi.
Proceeding laterally, the fibres of the corpus callosum radiate horizontally, downward and upward, this is called radiatio corporis callosi. Its anterior part with a smaller radius, located in the frontal lobes is called forceps minor, while the posterior part, with larger radius is the forceps major. The corpus callosum with its horizontal and downward fibres forms the roof of the lateral ventricle (with the exception of the cornu inferius), and it also forms its front in the cornu anterius and its sides in the cornu posterius et inferius. This thin lamina in the cornu posterius is called tapetum. The corpus callosum is only a secondary component of the third ventricle’s roof above the fornixes, velum interpositum and lamina choroidea epithelialis ventriculi tertii, while the thin lamina terminalis forms the anterior wall of the ventricle.
Under the corpus callosum, between the two crus fornicis we can find the commissura fornicis (fornix transversus, psalterium, lyra Davidis), which is the continuation of the fimbriae hippocampi rising from the hippocampus, and it connects the two hippocampuses. Thus it is sometimes referred to as commissura hippocampi. Sometimes a dilated CSF space is located between this and the corpus callosum, spreading into the cavum septi pellucidi. In this case we call it Verga’s ventricle.
The commissura anterior runs along in front of the above mentioned fornexes’ rostral columnae and behind the lamina terminalis. On frontal brain sections it can be visible when the downward and backward turning commissura anterior pierces through the capsula interna and the globus pallidus. It connects the two olfactory lobes.
The commissura habenularum, and under it, the commissura posterior is located in the posterior section of the third ventricle. The former connects the trigonum habenulae, joining the continuation of the two stria medullaris thalami. The corpus pineale hangs down from it. The commissura posterior, located on the diencephalon-mesencephalon border, plays a role in connecting functional systems such as the visual system (e.g. nucleus Edinger-Westphal, nucleus interstitialis Cajal) and the auditory-vestibular system (e.g. colliculi inferiores, vestibular nuclei), cervical spinal cord, oculomotor nuclei. Besides fibres it also contains neurons.
|
|