The brain is divided into four major areas: the cerebrum, the cerebellum, the brain stem, and the diencephalon. The brain stem consists of the midbrain, pons varolii, and medulla oblongata. The midbrain is further divided into the cerebral peduncle, corpora quadrigemina, and aqueduct of Sylvius (cerebral aqueduct). The diencephalon region includes the thalamus, hypothalamus, intermediate mass, third ventricle, and fornix.
The cerebrum is the largest part of the brain. The cerebral cortex is the outer layer of the cerebrum and is made up of gray matter. Internally the cerebrum is white matter. The cerebrum is divided into two cerebral hemispheres which are connected by a bridge of white matter called the corpus callosum. Each cerebral hemisphere has convolutions or raised folds on its surface that are referred to as gyri (sing. - gyrus). Separating the gyri are deep grooves called fissures and shallow grooves called sulci (sing. - sulcus). The most pronounced fissure is the longitudinal fissure, which almost separates the two cerebral hemispheres. Between the two cerebral hemispheres is an extension of the dura mater called the falx cerebri. Each cerebral hemisphere is divided into five lobes. They are the: frontal lobe, parietal lobe, temporal lobe, occipital lobe, and insula (island of Reil). The insula is a deep lobe and cannot be seen in an external view of the brain. It can be seen by separating the frontal lobe, parietal lobe, and temporal lobe.
The cerebellum is the second largest part of the brain and is inferior and posterior to the cerebrum. Externally it is gray matter and internally it is composed of white matter that is referred to as the arbor vitae (so named because it resembles the branches of a tree).
The midbrain is located inferior to the cerebrum and anterior to the cerebellum. The cerebral peduncles consist of a pair of cylindrical bodies that serve to connect the upper parts of the brain with lower parts of the brain and spinal cord. The corpora quadrigemina consists of four rounded lobes. The upper two are known as the superior colliculi and the lower two are known as the inferior colliculi. The aqueduct of Sylvius is a passageway that serves to connect the third ventricle and the fourth ventricle. The ventricles are discussed below. Located superiorly and slightly posterior to the corpora quadrigemina is the pineal gland.
The pons varolii or pons is located inferior to the midbrain and anterior to the cerebellum. It consists primarily of white matter. It serves to connect the spinal cord, cerebrum, and cerebellum.
The medulla oblongata or medulla is located inferior to the pons and superior to the foramen magnum. The medulla contains tracts (myelinated nerve fibers located within the central nervous system) that communicate between the spinal cord and various parts of the brain.
Figure 23.2 Lateral External View of the Brain
Cerebellum Frontal lobe Gyrus Medulla Occipital lobe Parietal lobe Pons Sulcus
The thalamus consists of two oval shaped structures located superior to the midbrain. It is primarily gray matter and it forms the lateral walls of the third ventricle. The two masses are connected by a bridge of gray matter known as the intermediate mass. The intermediate mass passes through the third ventricle.
The hypothalamus is located anterior and inferior to the thalamus. The hypothalamus forms the inferior floor and part of the lateral walls of the third ventricle. Anterior to the hypothalamus is the optic chiasma. In the optic chiasma, the medial halves of the optic nerves cross over and go to the opposite side of the brain. The lateral halves of the optic nerve do not cross over. The hypothalamus connects to the pituitary gland (hypophysis) by way of the infundibulum. The pituitary gland is located in the sella turcica of the sphenoid bone. Located inferior and posteriorly to the hypothalamus are the mammillary bodies. The two mammillary bodies are small and rounded.
Aqueduct of Sylvius Cerebellum Cerebral peduncle Cerebrum Choroid plexus of the fourth ventricle Choroid plexus of the third ventricle Corpora quadrigemina Corpus callosum Fornix Fourth ventricle Gyrus Hypothalamus Infundibulum Intermediate mass Mammillary body Medulla Optic chiasma Pineal gland Pituitary gland Pons Sulci Thalamus Third ventricle
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