ABSTRACT
The large cerebrum, divided (so it seems) into two separate portions, the cerebral hemispheres, is by far the most impressive structure of the human CNS and the one that most are referring to when speaking about “the brain�” In fact the “two” cerebral hemispheres are connected across the midline (see Figure 2�2A)� The cerebral hemispheres occupy most of the interior of the skull, the cranial cavity�
The brainstem is also seen from this perspective� It consists of three parts, from above downward the midbrain, pons, and medulla� The midbrain portion is usually obscured by the cerebral hemispheres in this view (see Figure 1�8 and Figure 3�2)�
The cranial nerves, which supply the structures of the head and neck, are attached to the brainstem� The brainstem and cranial nerves are considered in Chapter 3 (see Figure 3�4 and Figure 3�5)�
Part of the cerebellum, the “little brain,” can also be seen from this perspective� The cerebellum is introduced in Chapter 3 (see Figure 3�7 and Figure 3�8) and is further discussed with the motor system in Section 2 (see Figure 5�15, Figure 5�16, and Figure 5�17)�
This long extension of the CNS continues down from the medulla and is found in the vertebral canal (see Figure 1�2 and Figure 1�10)� The meninges, the connective tissue coverings of the spinal cord, have been opened, thereby showing the attached nerve roots (motor and sensory)� The spinal cord is discussed with Figure 1�10, Figure 1�11, and Figure 3�9� (The meninges, consisting of dura, arachnoid and pia, are discussed in Section 3�)
Note to the Learner: For safe handling of brain tissue, current guidelines recommend the use of disposable gloves when handling any brain tissue, to avoid possible contamination with infectious agents, particularly the so-called slow viruses� In addition, formalin is a harsh fixative and can cause irritation of the skin� Many individuals react to the smell of the formalin and may develop an asthmatic reaction� People who handle formalin-fixed tissue must take extra precautions to avoid these problems� In most laboratories, the brains are soaked in water before being put out for study�
This is the companion photograph to Figure 1�1 that is provided to assist the learner in visualizing the brain, brainstem, and spinal cord in situ�
The skull and brain have been cut in the midline, called a mid-sagittal view; therefore, one is looking at half of the brain� One can see that the cerebral hemispheres occupy most of the cranial cavity� The cerebral hemispheres are further discussed in Chapter 2�
Proceeding downward is a narrowed part of the central nervous system, the brainstem, which occupies the lower part of the cranial cavity; the pontine bulge is the most noticeable portion of the brainstem� The brainstem is further discussed in Chapter 3�
Behind the brainstem is the cerebellum, the so-called little brain� The brainstem and cerebellum occupy the posterior cranial fossa of the skull�
Additional features of the meninges of the brain and the way that the cranial cavity is partitioned are considered in Section 3�
The central nervous system continues downward as the spinal cord, situated in the vertebral canal� The spinal cord, in the adult human, terminates at the level of the upper lumbar vertebral level, whereas the vertebral canal continues (further explained with Figure 1�10)� The spinal cord and spinal canal are further discussed in this chapter and also in Chapters 2 and 3 (as well as in Section 2)�
The rest of the illustrations in this chapter are more detailed external views of the various parts of the central nervous system (CNS)�
Note to the Learner: The video of the cerebral hemispheres includes a demonstration of the exterior views of the brain hemispheres from the various perspectives (this chapter), as well as structures in the interior of the hemispheres (see Chapter 2)� This video can be accessed on the Web site (www�atlasbrain�com)�
It is also instructive at this stage to view the video on the Web site-the Interior of the Skull-which demonstrates how the brain “fits into” our skull�
When people talk about “the brain,” they are generally referring to the cerebral hemispheres, also called the cerebrum� The brain of higher apes and humans is dominated by the cerebral hemispheres� The outer layer, the cerebral cortex with its billions of neurons and its vast interconnections, is responsible for sensory perceptions, movements, language, thinking, memory, consciousness, and certain aspects of emotion� In short, the intact cerebral hemispheres are needed for all aspects of higher levels of function and to adapt to our constantly changing circumstances including socially and emotionally�
Most of the cerebral cortex is organized in six layers, known collectively as the neocortex, with the neurons of each layer having a different function� In formalin-fixed material, the neuronal cortex takes on a grayish appearance and is often referred to as the gray matter (see Figure 2�9A and Figure 2�10A)�
The surface of the hemispheres in humans (and in some other mammals) is thrown into irregular folds� This infolding allows an extensive expansion of the cerebral cortex to be contained within a restricted volume; otherwise, our skulls would have to be enormous!
