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Further, the great mass of seemingly-voluntary acts in such creatures as insects, worms, molluscs, are considered by physiologists to be as purely automatic as is the dilatation or closure of the iris under variations in quantity of light; and similarly exemplify the law, that an impression on the end of an afferent nerve is conveyed to some ganglionic centre, and is thence usually reflected along an efferent nerve to one or more muscles which it causes to contract.
Dresslar observed a thirteen months' old baby for four hours, and found, to follow Preyer's classification, impulsive or spontaneous, reflex, instinctive, imitative, inhibitive, expressive, and even deliberative movements, with marked satisfaction in rhythm, attempts to do almost anything which appealed to him, and almost inexhaustible efferent resources.
But in sleep the exciting influence of the brain is diminished, and the brain transmits much less of the normal excitement and normal tension to the spinal axis with its ramifications in the afferent and efferent nerves; in the waking state an external impression is promptly conveyed to the centres, whence it returns in corresponding movements with the usual connection and rapidity, whether reflex or deliberate.
A central or ganglion cell, which receives the sensory impulse, translates it into consciousness, and is the seat of whatever powers of perception, thought, or will the animal possesses. This also gives rise to the efferent or motor impulses, which are conveyed by a motor fibril to the corresponding muscle, exciting its contraction.
There are thus three main factors in the causation of this phenomenon: An increased nervous irritability; a local source of irritation; a ready efferent channel for nervous energy. Martin, who regards the phenomenon as normal, points out that when nausea and vomiting are absent or suddenly cease there is often reason to suspect something wrong, especially the death of the embryo.
The ventral-root divisions, of the fibers of mon-axonic neurons, the cell-bodies of which are in the gray matter of the cord. The first convey impulses to the cord and are called afferent neurons; the second convey impulses from the cord and are known as efferent neurons. A division of this pathway reaches the brain.
They receive lymph from the deep lymphatics of the lower limb, and some of the efferent vessels from the femoral and superficial inguinal glands. The lymph then passes on through the femoral canal to the external iliac glands.
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.
At the base of the inferior antennae in the Decapoda the so-called "green-gland" has its opening; in the Macrura at the end of a conical process. A similar conical process with an efferent duct traversing it is very striking in most of the Amphipoda.
It holds singularly aloof from other methods of efferent training and resists cooerdination with them, and its provisions for other than hand development are slight. It will be one of the last to accept its true but modest place as contributing certain few but precious elements in the greater synthesis that impends.
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