Cerebrum The cerebrum, by far the largest part of the human brain, consists of the cerebral hemispheres and the basal nuclei. The large, obvious cerebrum is divided into two halves, the right and left

The diencephalon, rostral to the midbrain and almost completely surrounded by the cerebral hemispheres, consists of four

major parts — the thalamus, subthalamus, epithalamus, and hypothalamus. Projecting from the hypothalamus is the hypophysis, or pituitary gland. The brainstem consists of the medulla oblongata, pons, and midbrain. The medulla resembles the spinal cord in many ways. Like the cord it gives rise to many nerve roots; however, these are the roots of cranial nerves rather than spinal nerves. The pons is the bridge between the two cerebellar hemispheres. The ventral portion of the pons forms a large synaptic relay station consisting of scattered gray centers called the pontine nuclei. The dorsal portion of the pons is more like the other regions of the brainstem, the medulla and midbrain. The midbrain sits just above the pons and is obscured by the large, overlapping cerebral hemispheres. It contains nuclei for cranial nerves III and IV, as well as ascending and descending fi ber tracts from the cerebrum.

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The brain sections on this and the following page depict aspects of brain anatomy that are not evident on the external views of the brain, and the association of the brain with surrounding structures of the head. Each section is approximately 2

centimeters thick and is an anterior view of three sections in succession. The fi rst section begins at the anterior aspect of the ear and the last section is just posterior to the ear.

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Frontal section of head at anterior aspect of auricle

Anterior view

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Developmentally the entire central nervous system forms from the hollow neural tube. As development proceeds and the wall of the neural tube becomes increasingly thicker, the hollow lumen of the tube

undergoes changes in relative size and shape throughout different regions of the changing central nervous system. As a result of this developmental history, there remains a hollow interconnected center throughout the entire central nervous system. This hollow core forms the ventricular system. Beginning within the cerebral hemispheres are the large paired lateral ventricles. Each lateral ventricle has a C-shape like its corresponding hemisphere. The lateral ventricles communicate via the interventricular foramina with a midline cavity, the third ventricle. The third ventricle sits within the core of the diencephalon where the right and left thalamus form its lateral walls. From the third ventricle a narrow channel, the aqueduct of the midbrain or cerebral aqueduct, passes through the core of the midbrain. This narrow channel expands in the region of the pons and cerebellum to form the fourth ventricle. The fourth ventricle tapers through the medulla to enter the spinal cord as the central canal. Within the four ventricles of the brain convoluted aggregations of capillaries, called a choroid plexus, project into the cavity of the ventricle. These capillary projections are the principal site for the production of cerebrospinal fluid.

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Sagittal section of braining revealing the ventricular system

Medial view, arrows show path of cerebrospinal fluid

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