The majority of white matter, on the other hand, develops at the lateral aspect of the occipital horn between the latter and the hemispheric convexity. The fibres originating from the occipital cortex and coursing within the occipital white matter can be divided into two groups. Amongst these groups, one can

BMN 673 supplier again subdivide three groups: i) fibres that extend to subcortical centres and are considered as projection fibres or corona radiata (Stabkranz) (Meynert); ii) other fibres have their terminations in cortical areas and are therefore association fibres. Association fibres either interconnect intralobal cortical areas (short association fibres), or link the occipital cortex with the cortex of a different lobe (long association fibres); iii) the third group crosses the inter-hemispheric midline and might terminate in [contralateral] CHIR-99021 supplier cortical or subcortical areas (callosal or commissural fibres). This mass of the occipital lobe fibres is not a tethered bundle, but is rather organised into bundles and layers according to certain rules. These layers can be distinguished based on their

direction, grouping and staining. The law of order is the following (Wernicke as cited above, p. 24): Every fibre reaches its destination via the shortest possible route, as far as this is in correspondence with embryological peculiarities of brain development. Thus, the following two conclusions can be reached: First, short fibres are located close to the cortex whilst longer fibres are located close to the ventricle. Second, fibres with roughly the same destination run in parallel or form bundles for a part of their common trajectory. A second, generally valid biological law not to be ignored in the study of brain structure is the law of variability. There are no two brains that are identical in all their details. Variability is

also observed in the arrangement and development of white matter anatomy. The cortex and the white matter are mutually dependent on each other. If a particular area of cortex is under-developed in a brain, then there Phosphatidylinositol diacylglycerol-lyase is also a paucity of fibres originating from this area. The occipital lobe fibres form four layers, which envelop the occipital horn like an onion skin from all sides except its opening. These layers, counted outwards from the medial to the lateral walls of the ventricles are (Fig. 3): 1. Layer of the corpus callosum: Forceps corporis callosi (1-10.), a.pars magna superior (1.), b. pars parva inferior (4.) This layer is found in the region of the parietal lobe. Another two bundles are closely located to the occipital lobe without joining its white matter system, namely: 5. Bogenbündel or oberes Längsbündel, fasciculus arcuatus see also longitudinalis superior [superior longitudinal fasciculus] Fibres originating from the occipital pole and surrounding areas bundle up in the middle of the white matter and run anterior-posteriorly.