The brain (cerebrum) is the more highly developed center of the nervous system (Figs 7-52 and 7-53). It lies in the cranial vault and is made up of five segments, divided according to their form and development:
- Cerebrum or endbrain (telencephalon)
- Midbrain (mesencephalon)
- Interbrain (diencephalon)
- Medulla oblongata (myelencephalon)
The first three parts together form the brainstem, which is structured like the spinal cord and contains the cell bodies of the cranial nerves as well as the centers for respiration and circulation.
The cerebrum or endbrain contains the stem ganglia; the two halves (hemispheres) of the brain with their cortices, which are linked by the corpus callosum; and the olfactory brain (rhinen-cephalon). Its purpose is to connect deeper-lying parts of the nervous system so that, as all sections of the nervous system interact, special brain functions of thinking, willing, and feeling can be achieved. In other words, the purpose of the cerebrum is to create consciousness.
The cerebrum weighs up to 1,400 g in adults and is made up of the two hemispheres whose surfaces are between 4.5 and 5.0 mm thick and consist of gray matter (Fig 7-54). This surface is known as the cortex. The cortical layer is shaped by numerous furrows (sulci) and convolutions (gyri) so that this layer can have a surface area of around 2,200 cm2. The number of convolutions is not a direct indication of the functional capacity of the brain.
In the cortical layer, the ganglion cells are concentrated as gray matter, while the white medullary substance, made up of afferent and efferent nerve fibers, is surrounded by the cortical layer. The contact surfaces between the gray and white brain matter are enlarged as a result of the deep furrowing of the cortex.
Almost 200 centers of sensory perception and response to stimuli can be identified on the cortical layer, and these are described as the visual, tactile, speech, and motor centers. The individual cortical centers differ in their fibrous and cellular structure so that six or seven layers of cortex can be found.
Topographically, the midbrain (mesencephalon) can be precisely identified, but it does not have a precise function of its own. However, it does regulate motor functions of posture and position in humans. It coordinates the instructions from the higher centers of the brain within the motor system.
The interbrain lies between the two hemispheres and is composed of the thalamus, the hypothalamus, and the extrapyramidal system. The interbrain (diencephalon) is an important relay center for nearly all stimuli from the skin, eyes, and ears as well as other parts of the brain.
As the main mass in the diencephalon, the thalamus is an independent coordinating center in which all sensory pathways from the spinal cord are "switched over" to the cerebral cortex. It is here that feelings of pleasure, dislike, pain, and fear are triggered and conducted to the cerebrum.
The hypothalamus is the center of the autonomic nervous system and, with the adjacent pituitary gland (or hypophysis), controls the endocrine system. The hypothalamus regulates body temperature, water and sugar balance, fat and mineral metabolism, and especially sleeping and waking states. The extrapyramidal system regulates the voluntary actions by controlling muscle tone.
The cerebellum is a segment of the brain lying in the dorsal part of the cranium. It is the integrating organ for programming and coordinating the movement of striated muscles and for maintaining muscle tone and balance.
The medulla oblongata (myelencephalon) is an extremely complicated part of the central nervous system. The control centers for waking functions are located here; these centers protect the other parts of the brain against overstimulation. This is where vital control centers (eg, for respiration or cardiac activity) and reflex control centers (eg, vomiting, swallowing) are located.