The planorbular, white shell is cancellated with elevated, decussating tansverse and longitudinal lines. It is very widely umbilicated. The spire is excavated. The whorls are rapidly increasing, spirally striated, with a sloping, smooth sutural margin.
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They are closely encircled with decussating lirae, a faint straw-coloured or golden tinge being sometimes observable on them. The body whorl is fairly tumid. The smallish aperture is oblong. The outer lip is thin.
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The length of the shell varies between 6 mm and 24 mm. The shell is cancellated with decussating striae. The sinus is rather broad. The color of the shell is pale fulvous with two revolving rows of short flames or spots of chestnut.
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The length of the shell attains 11 mm, its diameter 4 mm. This thin, inflated, fusiform shell is almost colourless. It contains 8-9 whorls, of which four in the protoconch. These are ochreous and show a decussating sculpture of the finest character.
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Most of the trigonum habenulæ's fibers are, however, directed downward and form a bundle, the fasciculus retroflexus of Meynert, which passes medial to the red nucleus, and, after decussating with the corresponding fasciculus of the opposite side, ends in the interpeduncular nucleus.
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The length of the shell varies between 24 mm and 55 mm. The shoulder somewhat flatter than in Genota mitriformis. The nodules are more distinct, as are the longitudinal lines or folds. The revolving sculpture is scarcely decussating them and sometimes becomes obsolete.
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The dentatothalamic tract (or dentatorubrothalamic tract) is a tract which originates in the dentate nucleus and follows the ipsilateral superior cerebellar peduncle, decussating later on and reaching the contralateral red nucleus and the contralateral thalamus.Operative Neurosurgery: Dentatorubrothalamic tract . The term "dentatorubrothalamocortical" is sometimes used to emphasize termination in the cerebral cortex.
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The trapezoid body is a bundle of decussating fibers in the ventral pons that carry information used for binaural computations in the brainstem. Some of these axons come from the cochlear nucleus and cross over to the other side before traveling on to the superior olivary nucleus. This is believed to help with localization of sound.
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The optic nerve leaves the orbit (eye socket) via the optic canal, running postero- medially towards the optic chiasm, where there is a partial decussation (crossing) of fibers from the temporal visual fields (the nasal hemi-retina) of both eyes. The proportion of decussating fibers varies between species, and is correlated with the degree of binocular vision enjoyed by a species.Textbook of Veterinary Anatomy, 4th Edition.
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It also contains decussating fibers from the olfactory tracts, vital for the sense of smell and chemoreception. The anterior commissure works with the posterior commissure to link the two cerebral hemispheres of the brain and also interconnects the amygdalae and temporal lobes, contributing to the role of memory, emotion, speech and hearing. It also is involved in olfaction, instinct, and sexual behavior. In a sagittal section, the anterior commissure is oval in shape, having a long vertical axis that measures about 5 mm.
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The first-order neurons (from the trigeminal ganglion) enter the pons and synapse on second-order neurons in the principal (chief sensory) nucleus. Axons of the second-order neurons then decussate to enter the trigeminal lemniscus in the midbrain and then ascend to the ventral posteromedial nucleus of the contralateral thalamus, forming the ventral trigeminothalamic tract. A subset of these fibers do not decussate and travel to the ipsilateral ventral posteromedial nucleus of the thalamus. These non-decussating fibers give rise to the dorsal trigeminothalamic tract.
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The portion of the midbrain from where this tract originates is the superior colliculus, which receives afferents from the visual nuclei (primarily the oculomotor nuclei complex), then projects to the contralateral (decussating dorsal to the mesencephalic duct) and ipsilateral portion of the first cervical neuromeres of the spinal cord, the oculomotor and trochlear nuclei in the midbrain and the abducens nucleus in the caudal portion of the pons. The tract descends to the cervical spinal cord to terminate in Rexed laminae VI, VII, and VIII to coordinate head, neck, and eye movements, primarily in response to visual stimuli.
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The habenular trigone is a small depressed triangular area above the superior colliculus and on the lateral aspect of the posterior part of the taenia thalami. Underlying this area is the habenula. Fibers enter it from the stalk of the pineal gland, and others, forming what is termed the habenular commissure, pass across the middle line to the corresponding ganglion of the opposite side. Most of its fibers are, however, directed downward and form a bundle, the fasciculus retroflexus of Meynert (or habenulointerpeduncular tract), which passes medial to the red nucleus, and, after decussating with the corresponding fasciculus of the opposite side, ends in the interpeduncular nucleus.
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The fibers of the anterior commissure can be traced laterally and posteriorly on either side beneath the striatum into the substance of the temporal lobe. It serves in this way to connect the two temporal lobes, but it also contains decussating fibers from the olfactory tracts, and is a part of the neospinothalamic tract for pain. The anterior commissure also serves to connect the two amygdalae. The corpus callosum allows for communication between the two hemispheres and is found only in placental mammals (the eutherians), while it is absent in monotremes and marsupials, as well as other vertebrates such as birds, reptiles, amphibians and fish.
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