The largest part of the brain, divided into two hemispheres, is the [1] cerebrum . It is marked by numerous twisted folds called [2] gyri separated by shallow grooves called [3] sulci . The second-largest part of the brain, the [4] cerebellum , is located at the rear of the cranial cavity beneath 1. If 1 and 4 are removed, there remains a stalklike [5] brainstem . In brain anatomy, directions are often given by the terms [6] rostral , meaning "toward the nose," and [7] caudal , meaning "toward the tail." At 4 weeks, a human embryo shows a hollow channel called the [8] neural tube along the dorsal side. This channel then begins to dilate anteriorly and differentiate into major regions of the future brain. These include the [9] forebrain , midbrain, and [10] hindbrain .

Meninges, Ventricles, Cerebrospinal Fluid, and Blood Supply

The meninges are three fibrous membranes that envelop the CNS. The outermost meninx is the tough [11] dura mater . The middle layer is a more loosely organized [12] arachnoid mater , and the inner meninx is a thin, translucent [13] pia mater that closely follows the contours of the brain and spinal cord. The brain and spinal cord are hollow and contain a liquid called [14] cerebrospinal fluid , which is secreted by capillary networks called the [15] choroid plexuses . The four fluid-filled cavities of the brain are called the [16] ventricles . Fluid is secreted into these by 15 and eventually flows out of the [17] fourth ventricle to bathe the external surfaces of the brain and spinal cord. From there, the fluid is absorbed by [18] arachnoid villi , which are extensions of the middle meninx. Although the brain is richly supplied with blood vessels, there is a [19] blood-brain barrier that strictly regulates what substances can exit the bloodstream and get into the brain tissue. This is lacking from regions called the [20] circumventricular organs (CVOs) , where such pathogens as HIV can invade the brain.

The spinal cord extends from the foramen magnum to vertebra [21] L1 . It gives rise to 31 pairs of [22] spinal nerves at regular intervals along its length. The gray matter of the cord is arranged in a butterfly-like shape with two [23] dorsal horns , which receive sensory input, and two [24] ventral horns where motor output originates. The white matter is divided into six major bundles called [25] columns, or funiculi , and each of these has several smaller subdivisions called [26] tracts, or fasciculi . Ascending 26s carry information up the cord to the brain. Two of these are the [27] gracile fasciculus and the [28] cuneate fasciculus, which form the entire dorsal column. The [29] descending 26s carry signals down the cord and have names ending in -spinal.

During embryonic development, the myencephalon develops into the [30] medulla oblongata . This contains nuclei for cardiovascular and respiratory control, gives rise to the last four cranial nerves, and conducts signals forward to the next part of the brainstem, the [31] pons . This region has nuclei that control sleep, posture, respiration, and urination, among other functions. This and the larger [32] cerebellum are the only two structures derived from the embryonic metencephalon. The latter structure consists of two hemispheres connected by the narrow [33] vermis . It is concerned with motor skills and modulation of skeletal muscle contractions. Continuing forward from 30, the next part of the brainstem is the [34] midbrain , which includes nuclei concerned with auditory and visual reflexes and other eye movements. The brainstem regions up to this point contain a group of more than 100 nuclei, collectively called the [35] reticular formation , which are concerned with autonomic functions, pain modulation, alertness, and arousal.

The largest part of the diencephalon is the [36] thalamus , which bulges into the lateral ventricle under each cerebral hemisphere. It acts as a relay point for nearly all signals going from lower parts of the brain to their destinations in the [37] cerebral cortex . Inferior to 36 is the funnel-like [38] hypothalamus , which forms part of the walls and floor of the [39] third ventricle. Among other things, it contains a [40] hypothalamic, or thermostat , which controls body temperature, and neurons called [41] osmoreceptors , which monitor the osmolarity of the blood. The pineal gland and habenula constitute a small superior part of the diencephalon called the [42] epithalamus .

The gray matter forms a surface layer called the cerebral [43] cortex . This is divided into five lobes—a deep [44] insula , which cannot be seen at the brain surface, and four others named for the bones overlying them. Of these four lobes, voluntary motor control and social judgment are located in the [45] frontal lobe, visual signals are received in the [46] occipital lobe, hearing is associated with the [47] temporal lobe, and most other senses such as touch and pain throughout the body are processed in the [48] parietal lobe. Most of the cerebrum is composed of white matter, which consists of three kinds of [49] tracts that carry signals from one part of the brain to another. Those that communicate between the right and left cerebral hemispheres pass mainly through a large C-shaped corpus [50] callosum . The most characteristic neurons of the gray matter are triangular-looking [51] pyramidal cells. In regions called the [52] neocortex , these neurons and glial cells form six histologically recognizable layers. Gray matter forms not only the surface of the cerebrum, but also a deep cluster of three or more [53] basal nuclei in each hemisphere concerned mainly with motor control. Many primitive emotional functions reside in the [54] limbic system , a ring of structures on the medial border of the temporal lobe and encircling the corpus callosum and thalamus

Neurological disorders can often be diagnosed with the aid of a/an [55] electroencephalogram , a recording of the brain waves. These brain waves show predictable changes as a person falls asleep, with delta waves dominating in the deepest stage, called [56] stage 4, or slow-wave sleep. Most dreaming occurs during a stage called [57] REM sleep because of flickering movements of the eyes that occur then. Voluntary muscle contractions originate with neurons in the [58] precentral gyrus of the cerebrum, which forms the posterior limit of the frontal lobe. Somatic sensation resides in a corresponding [59] postcentral gyrus just posterior to this. Both of these gyri exhibit [60] somatotropy , an inverted "map" of the opposite side of the body. The special senses such as smell, vision, and hearing project to other specialized regions of the cerebral cortex. In each case, the primary sensory cortex is adjacent to a/an [61] association area of neurons that interpret the sensory information. The comprehension of written and spoken language resides in [62] WernickeÕs area slightly anterior to the visual cortex. This area is also where one decides what to say. [63] BrocaÕs area of the inferior frontal cortex generates a motor program, or plan, to produce speech. Lesions to these areas cause various forms of language defects called [64] aphasia . Memory is a complex function involving multiple regions of the cerebrum. The two cerebral hemispheres look similar but are specialized for different functions. This division of labor between the right and left sides is called cerebral [65] lateralization .