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The reptilian complex, also known as the R-complex or "reptilian brain" was the name MacLean gave to the basal ganglia, structures derived from the floor of the forebrain during development. The term derives from the fact that comparative neuroanatomists once believed that the forebrains of reptiles and birds were dominated by these structures. MacLean proposed that the reptilian complex was responsible for species typical instinctual behaviors involved in aggression, dominance, territoriality, and ritual displays.
The main components of the basal ganglia are the striatum, the globus pallidus, the substantia nigra, and the subthalamic nucleus. The basal ganglia are a collection of distinct masses of gray matter lying deep in the brain not far from the junction of the thalamus.
The largest component, the striatum, receives input from many brain areas but sends output only to other components of the basal ganglia. The pallidum receives input from the striatum, and sends inhibitory output to a number of motor-related areas. The substantia nigra is the source of the striatal input of the neurotransmitter dopamine, which plays an important role in basal ganglia function. The subthalamic nucleus receives input mainly from the striatum and cerebral cortex, and projects to the globus pallidus
The paleomammalian brain consists of the septum, amygdala, hypothalamus, hippocampal complex, and cingulate cortex. MacLean first introduced the term "limbic system" to refer to this set of interconnected brain structures in a paper in 1952. MacLean's recognition of the limbic system as a major functional system in the brain has won wide acceptance among neuroscientists, and is generally regarded as his most important contribution to the field. MacLean maintained that the structures of the limbic system arose early in mammalian evolution (hence "paleomammalian") and were responsible for the motivation and emotion involved in feeding, reproductive behavior, and parental behavior.
The neomammalian complex consists of the cerebral neocortex, a structure found uniquely in mammals. MacLean regarded its addition as the most recent step in the evolution of the mammilian brain, conferring the ability for language, abstraction, planning, and perception.
The unusual surface appearance of the cerebellum conceals the fact that most of its volume is made up of a very tightly folded layer of gray matter, the cerebellar cortex. It has been estimated that, if the human cerebellar cortex were completely unfolded, it would give rise to a layer of neural tissue about 1 meter long and averaging 5 centimeters wide — a total surface area of about 500 square cm, packed within a volume of dimensions 6 cm × 5 cm × 10 cm. Underneath the gray matter of the cortex lies white matter, made up largely of myelinated nerve fibers running to and from the cortex. Embedded within the white matter — which is sometimes called the arbor vitae (Tree of Life) because of its branched, tree-like appearance in cross-section — are four deep cerebellar nuclei, composed of gray matter
[The Thalamus’] function includes relaying sensory and motor signals to the cerebral cortex, along with the regulation of consciousness, sleep, and alertness. The thalamus has multiple functions. It may be thought of as a kind of switchboard of information. It is generally believed to act as a relay between a variety of subcortical areas and the cerebral cortex. In particular, every sensory system (with the exception of the olfactory system) includes a thalamic nucleus that receives sensory signals and sends them to the associated primary cortical area.
The thalamus also plays an important role in regulating states of sleep and wakefulness. Thalamic nuclei have strong reciprocal connections with the cerebral cortex, forming thalamo-cortico-thalamic circuits that are believed to be involved with consciousness. The thalamus plays a major role in regulating arousal, the level of awareness, and activity. Damage to the thalamus can lead to permanent coma.
The thalamus has been thought of as a "relay" that simply forwards signals to the cerebral cortex. Newer research suggests that thalamic function is more selective
Kayumov et al. showed that light containing only wavelengths greater than 530 nm does not suppress melatonin in bright-light conditions. Use of blue-blocking goggles the last hours before bedtime has also been advised for people who need to adjust to an earlier bedtime, as melatonin promotes sleepiness.
In non-human mammals it is a large structure. In humans this mass averages about 1 cm in length in its antero-posterior diameter. It sometimes consists of two parts and occasionally is absent. The interthalamic adhesion is found in 70-80% of humans. It is present more often in females and larger than in males by an average of 53 percent.  When absent in development, no noticeable deficit has been observed.
In 1889, a Portuguese anatomist by the name of Macedo examined 215 brains, showing that male humans are approximately twice as likely to lack an interthalamic adhesion as are female humans. He anecdotally attributed the finding to a "prevailing feature of people deprived of [the interthalamic adhesion] is to present in their psychical acts a remarkable precipitation, joined to a certain dysharmony between internal and external feelings." It is noted to be enlarged in patients with Chiari II malformations. 
The interthalamic adhesion thickness in both T1- and T2-weighted transverse images were measured in all dogs. The interthalamic adhesion thickness in the normal and demented groups was 6.79 +/- 0.70 and 3.82 +/- 0.79 mm, respectively. The interthalamic adhesion thickness in the demented group was significantly smaller […]suggest that interthalamic adhesion thickness may be a good parameter for evaluating brain atrophy in dogs with cognitive dysfunction.