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.
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.
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 Ventricles Of The Brain
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.