Figure 7-1
Ventricles of the brain.
Ventricles of the brain.
Before we begin to explore the human brain, let's review a few anatomical ideas so we can all begin on common ground.
The brain is uniquely human and can be divided into a large cerebrum with two hemispheres, a cerebellum also with two hemispheres, and a brain stem.
Embryonically, the central nervous system (CNS) begins as a hollow cellular tube. During this stage of rapid cell growth, nerve tissue folds on itself and creates the variety of interwoven parts of the CNS. There are several adult remnants of this once hollow cellular tube and the subsequent cellular growth and infolding. The ventricular system is one adult vestige of that hollow tube. It is a series of interconnected cavities (ventricles) filled with cerebrospinal fluid (CSF). (Figure 7-1)
There are four ventricles, two lateral (ventricles one and two), one per cerebral hemisphere. Ventricle three is near the middle of the brain stem and a fourth ventricle found posterior to the brain stem and anterior to the cerebellum. The ventricular system of the brain is in free communication with the subarachnoid space surrounding the CNS. CSF produced in the ventricular system can circulate from the ventricular system to the subarachnoid space.
Each cerebral hemisphere is composed of an inner core of white matter (myelinated nerve cell processes or fibers), surrounded by an outer layer of gray matter (nerve cell bodies). This outer layer is known as the cerebral cortex (cortex = bark), and is characterized by highly folded curves and windings. The ridges of these folds are called gyri (gyrus singular), and the grooves are called sulci (sulcus singular). Very deep sulci are called fissures. Gyri and sulci result from prolific embryonic cell growth.
Each cerebral hemisphere is further subdivided into four lobes (frontal, parietal, occipital and temporal) separated by sulci and fissures. The two cerebral hemispheres are connected by a nerve fiber bundle known as the corpus callosum. Each hemisphere is believed to have a somewhat different function. The cerebrum is responsible for conscious sensory and motor activity along with cognizant thought such as learning, memory, emotions and intellect. Human cerebrums have become so large they literally have grown over our face (Figure 7-2).
The cerebellum is often referred to as the ancient brain, because it unconsciously controls muscle activity and coordination. The cerebellum also has two hemispheres. Both the cerebrum and the cerebellum are connected to the brain stem.
The brain stem can be divided into four parts: the paired diencephalon on the sides of the third ventricle, the mesencephalon or midbrain, the pons and the medulla oblongata. It connects the spinal cord to the cerebrum and cerebellum and it houses clusters of motor and sensory neurons, called nuclei, that regulate and integrate information coming into and out from the CNS. (Figure 7-3).