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The cell-bodies of the inhibitory fibers are located in the bulb, from where their fibers pass to the heart as a part of the vagus nerve. In addition to the fibers above mentioned, are those that convey impulses from the heart to the bulb.
In a somewhat similar manner, heat leads to a relaxation of the arterial walls and an increase in the blood supply to the skin. Other changes in the blood supply to different parts of the body are also of the nature of reflex actions. As in the work of digestion, neurons having their cell-bodies in the sympathetic ganglia aid in the control of the circulation.
It is divided into three lobes—a central lobe and two lateral lobes—and weighs about two and one half ounces. In its general form and appearance, as well as in the arrangement of its cell-bodies and axons, the cerebellum resembles the cerebrum.
Like all of the other organs of lesser importance belonging to the nervous system, the brain is a complex of tissues which in the last analysis are groups of cell-bodies with their fibrous prolongations. When these cellular elements are in operation, mental processes go on; the unit of the mental process therefore is the functioning of a brain-cell.
It may be said, in general terms, that the nervous centres, the brain and the spinal cord, which are parts of one anatomical whole, are characterized by the presence of the cell-bodies as well as their extensions, while nerves consist only of the extensions or arms of the cell-bodies.
*The Pons*, or pons Varolii, named from its supposed resemblance to a bridge, is situated in front of the cerebellum, and is readily recognized as a circular expansion which extends forward from that body. It consists largely of bands of nerve fibers, between which are several small masses of cell-bodies. The fibers connect with different parts of the cerebellum and with parts above. C. Cerebrum.
Cb. Cerebellum. M. Midbrain. P. Pons. B. Bulb. I-XII. Cranial nerves. *The Bulb*, or medulla oblongata, is, properly speaking, an enlargement of the spinal cord within the cranial cavity. It is somewhat triangular in shape, and lies immediately below the cerebellum. It contains important clusters of cell-bodies, as well as the nerve fibers that pass from the spinal cord to the brain.
*The Cerebrum.*—The cerebrum comprises about seven eighths of the entire brain, and occupies all the front, middle, back, and upper portions of the cranial cavity, spreading over and concealing, to a large extent, the parts beneath. The surface layer of the cerebrum is called the cortex. This is made up largely of cell-bodies, and has a grayish appearance.
A. Association fibers. C. Commissural fibers. P. Projection fibers. The cell-bodies with which the fiber bundles connect are in the surface layer or cortex.
In fact, the olfactory cells resemble closely the cell-bodies of neurons, and are thought to be such. *The Olfactory Stimulus.*—Only substances in the gaseous state can be smelled. From this it is inferred that the stimulus is supplied by gas particles. Solids and liquids are smelled because of the gas particles which separate from them.
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