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According to Head and his co-workers, Sherren and Rivers, the afferent fibres in the peripheral nerves can be divided into three systems: The fibres of this system run with the motor nerves, and pass to muscles, tendons, and joints. Even division of both the ulnar and the median nerves above the wrist produces little loss of deep sensibility, unless the tendons are also cut through.
What ones contain both afferent and efferent fibers? In what respects is the nervous system similar to a system of telephony? In what respects is it different? Observe the convolutions of the cerebrum and compare these with the parallel ridges of the cerebellum. Observe the appearance and general distribution of the nerves and their connection with the brain and spinal cord.
A complex system of intermediate neurons, found mostly in the brain, join the afferent with the efferent pathways. *Choice in Voluntary Action.*—In reflex action a given stimulus, acting in a certain way; produces each time the same result. This is not the case with voluntary action, the difference being due to the mind.
State the purpose of the nervous impulse. Show that the exciting cause of bodily action is outside of the nervous system and, to a large extent, outside of the body. Describe the arrangement that enables stimuli outside of the body to cause action within the body. Describe a reflex action and show how it is brought about. Distinguish between afferent, efferent, and intermediate neurons.
The exact location of an abscess also may furnish a key to its source; in axillary abscess, for example, if the suppuration is in the lymph glands the infection has come through the afferent lymphatics; if in the cellular tissue, it has spread from the neck or chest wall; if in the hair follicles, it is a local infection through the skin.
From spoken language man has advanced to written language, so that the sight of the written or printed word also arouses in the mind the associated idea. *The Ear* is the sense organ which enables sound waves to so act upon afferent neurons as to excite impulses in them. The effect upon the mind which these impulses produce is known as the sensation of hearing.
Most of these have their cell-bodies in the gray matter of the cord, while their fibers pass into the spinal nerves by the ventral roots. They may be stimulated by impulses either from the intermediate neurons, or from branches of the afferent neurons. Their impulses reach and stimulate the muscles.
In these cases, therefore, reflex action must be effected through the brain, all the nerves involved being cerebral. 'When the whole body starts at a loud noise, the afferent auditory nerve gives rise to an impulse which passes to the medulla oblongata, and thence affects the great majority of the motor nerves of the body.
From the sentient surface an afferent impulse passes along to the posterior root of the spinal cord, the nerve fibers of the posterior root ending in minute filaments among the small cells of this part of the cord . In some unknown way this impulse passes across the gray part of the cord to the large cells of the anterior root , the cells of this part being connected by their axis-cylinder with the efferent fibers . These convey the stimulus to the fibers of the muscle , which accordingly contract.
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