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Essentials Of Human Anatomy



The Central Nervous System

ch.1
p.25

The Brain is composed of the Cerebrum, Cerebellum, and BrainStem housed within the Cranial cavity. The Cerebrum consists of two Hemispheres seperated by the Longitudinal Fissure.

The Hemispheres are connected by a large C-shaped bundle of fibers carrying impulses between them, the Corpus Callosum and by the BrainStem.

The surface of the Hemispheres are convoluted with folds, Gyri, seperated from each other by elongate depressions, called Sulci or fissures. The Gyri and Sulci have a relatively stable pattern from brain to brain and thus most of them have specific names.

Almost at right angles to the Longitudinal Fissure, crossing the Cerebrum laterally and downward, is the Central Sulcus, Below the end of this Sulcus is the Horizontal Lateral Fissure.

p.26

Each Hemispheres is divided into several Lobes, whose names correspond to the bones of the skull lying superfical to them. The Frontal Lobe lies above the Lateral Fissure in front of the Central Sulcus.

Behind the Central Sulcus is the Parietal Lobe and behind that the Occipital Lobe, although there is no specific boundry between them laterally. Medially they are seperated by the Parieto-Occipital Fissure. Below the Lateral Fissure and anterior to the Occipital is the Temporal Lobe.

The Cerebrum has an outer layer of gray matter, composed primarily of nerve cells, called the Cortex. Below this layer lie large bundles of nerve cell processes, or fibers, the White Matter.

Embedded deep within the White Matter are the Basal Ganglia, or Corpus Striatum, a group of nuclie which serve to coordinate Motor and Sensory Impulses.

The Cortex is functionally organized. Immediately anterior to the Central Sulcus lies the PreCentral Gyrus, the center for voluntary motor movements.

Immediately posterior is the Somatic Sensory area, or Postcentral Gyrus, set aside for conscious perception of general Sensory phenomena. Above and below the Calcarine Sulcus on the medial side of the Occipital Lobe are the Cortical areas for Vision.

p.26

Auditory phenomena are localized to the upper part of the Temporal Lobe, opposite the Somatic Sensory area.

Smell, or Olfactory sensation are associated with the inferior surface of the Temporal Lobe (Tentorial), although the Olfactory Nerve (Cranial Nerve I), ends in the inferior portion of the Frontal Lobe.

Most of the Cortex is occupied by Association Areas, which are devoted to intergration of Motor and Sensory phenomena, advanced intellectual activities, such as abstract thinking, comprehension and execution of language, and memory storage and recall.

In keeping with embryological development the Cerbral Hemispheres are hollow, each containing a Lateral Ventricle.

The Ventricles contain a vascular membrane, the Choroid Plexus, that secretes CerebroSpinal Fluid. The Lateral Ventricles communicate with the cavity of the DienCephalon, the midline Third Ventricle, by way of InterVentricular Foramina.

p.27

The DienCephalon, the most rostral part of the BrainStem, is deeply embedded in the inferior aspect of the Cerebrum situated on either side of the slit-like Third Ventricle.

A thin membrane and attached Choroid Plexus, the Tela Choroidea, roofs this cavity.

The inferior portion of the DienCephalon is the only part visible on the intact Brain and is found in the space between the Optic Chiasm anteriorly and the Cerebral Peduncles posteriorly.

This forms the Floor of the Third Ventricle and is named the HypoThalamus in relation to the Thalamus, the largest part of the DienCephalon, which lies above it. Projecting from the HypoThalamus on a slender stalk, or Infundibulum, is the Hypophysis.

This is seldom seen on Brain specimens, since its Meninges hold it tightly in the Sella Tursica of the skull when the Brain is removed from the body.

The HypoThalamus and Hypophysis are closely related and regulate many important body functions, such as Temperature, Water and Fat Metabolism, Sleep, Sexual Activity and Emotional Control.

The Thalamus receives nearly all sensory impulses from the Peripheral Nervous System and relays them to the Cerebral Cortex.

The Optic Nerve (Cranial Nerve II), in reality a tract of the Brain, is closely associated with the DienCephalon.

Fibers from the Nasal half of each Nerve cross to the opposite side in the Chiasm, then form the Optic Tracts which sweep lateral to the HypoThalamus and Cerebral Peduncles to end in the Thalamus and MesenCephalon.

p.28

The MesenCephalon (MidBrain) is the smallest part of the BrainStem, being about 2cm in length. Its narrow cavity, the Cerebral Aqueduct connects the Third and Fourth Ventricles.

Inferiorly, the Cerebral Peduncles are prominent fiber bundles connecting centers above and below the MesenCephalon. Dorsally, two Superior and two Inferior Colliculi, collectively referred to as Corpora Quadrigemina, are found.

These are relay centers in the Optic and Auditory Systems, respectively.

The Nuclei of the Oculomotor (III) and Trochlear (IV) Nerves and part of the Trigeminal Nerve (V) lie in the MidBrain.

The Oculomotor Nerve arises from between the Cerebral Peduncles and the Trochlear from the dorsal side of the MidBrain. Several other important Nuclei such as the Red Nucleus and the Substantia Nigra are found here as well.

p.28

Caudal to the MesenCephalon lie the Pons ventrally and the Cerebellum dorsally, with the Fourth Ventricle situated between them. The Pons consists superfically of large transverse fiber bundles which connect the two Cerebellar Hemispheres.

Deep within the Pons lie longitudinal fiber bundles, which carry impulses up and down the BrainStem, and scattered Nuclei.

The Nuclei of the Trigeminal (V), Abducens (VI), Facial (VII), and Vestibulocochlear (VIII) Nerves are located dorsally in the Pons. Their respective Nerves exit from the Lateral and Inferior parts of the organ.

p.29
The lowest part of the BrainStem, below the Pons, is the Medulla Oblongata. It is continuous with the Spinal Cord just above the First Cervical Spinal Nerve, but the boundary is indistinguishable.

Structures contained in the Medulla extend into the Spinal Cord with a gradual rearrangement in course.

The Medulla transmitts all fibers connectiong Brain and Spinal Cord. Nuclei of the last four Cranial Nerves: Glossopharyngeal (IX), Vagus (X), Accessory (XI), and Hypoglossal (XII) are located here.

The first three emerge laterally as a continuous series of rootlets that coalesce into individual nerves. Part of the Accessory Nerve (Spinal portion) arise from the Cervical Spinal Cord as well.

The Hypoglossal Nerve begins more ventrally, seperated from the others by an oval bulge called the Olive.

Lying in the Medulla are centers regulating important functions such as the Respiratory Center, Cardiac Center, Vasomotor Center, and centers for Swallowing, Gastric secretion and Sweating.

In contrast to the Cerebrum, the Cerebellum is a solid mass of tissue. Like the Cerebrum, it is covered by a layer of Gray Matter, the Cortex, overlaying White Matter and the surface is thrown into a series of parallel folds, here called Folia. It has two Hemispheres, a Midline Vermis and several nuclei internally.

Three sets of Peduncles, lying Superior, Lateral and Inferior to the Fourth Ventricle, connect the Cerebellum to the MesenCephalon, Pons and Medulla Oblongata.

The Cerebellum is a Coordination Center for Muscular Activity, particularly walking. It is the only part of the CNS that does not give rise to Peripheral Nerves.


The Ventricles Of The Brain

In keeping with embryonic development, the Cerebral Hemispheres are hollow, each containing a Lateral Ventricle. The Ventricles contain a vascular membrane, the Choroid Plexus, that secretes CerebroSpinal Fluid.

The Lateral Ventricles communicate with the cavity of the DienCephalon, the Midline Third Ventricle, by way of the InterVentricular Foramina.

A thin membrane and attached Choroid Plexus roofs the Third Ventricle. In the MidBrain, the narrow Cerebral Aqueduct connects the Third and Fourth Ventricles.

The Fourth Ventricle is located between the Pons, Cerebellum and Medulla. It communicates with the Cerebral Aqueduct, the Central Canal of the Spinal Cord and the SubArachnoid space which surrounds the Central Nervous System.

The roof of the Fourth Ventricle caudal to the Cerebellum, the Tela Choroidea, is thin like that of the Third Ventricle and has a Choroid Plexus.

It is perforated by a small median aperture and two lateral aperatures that allow CerebroSpinal Fluid to exit the Ventricular System and bath the Brain and Spinal Cord.

CerebroSpinal Fluid is a watery, alkaline fluid, similar in constitution to blood plasma. It is elaborated by or through the Choroid Plexuses of the Lateral, the Third and the Fourth Ventricles of the Brain.

It occupies the InterCommunicating Ventricles and, being constantly formed, is drained from the Ventricles by minute Foramina in the roof of the Fourth Ventricle.

These are the Median and Lateral Apertures of the Fourth Ventricle, the latter pair being located at the extremities of the Lateral recesses of the Ventricle.

Small additions to the CerebroSpinal Fluid are made through the PeriVascular channels of the Brain surface and by the Ependyma of the Central Canal of the Spinal Cord. The total volume of the Fluid is from 130 to 150 cc.

p.30

Emerging through the Foramina into the SubArachnoid Space, the CerebroSpinal Fluid baths the surface of the Brain and Spinal Cord, providing a fluid suspension and a valuable shock absorber around these organs of the Nervous System.

The Fluid has a pressure of about 100 mm of water, which is intermediate between that of the Peripheral Arterial and Venous Sinus pressure.

CerebroSpinal Fluid readily passes through the thinned out membrane of the Arachnoidal granulations and the Endothelial lining of the Dural Sinuses and joins the Venous Blood of the Sinus. A smaller part of the Fluid is returned to the Vascular System by way of the Lymphatics of the Cranial Nerves.

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