. The elementary nervous system . nounced polarity, for contraction normally begins in the region of the sinus and progresses thence in sequence over the auricle, the ventricle and the bulb. The fact that by appropriate stimulation this sequence may be re- versed gives the heart a more diffuse character than the small intestine shows. But the uncertainty as to the myogenic or neurogenic na- ture of its beat leaves the heart a less clear example of these peculiarities than some other instances. As diffuse transmission is characteristic of the nerve-net and polarized transmission is a feature of
. The elementary nervous system . nounced polarity, for contraction normally begins in the region of the sinus and progresses thence in sequence over the auricle, the ventricle and the bulb. The fact that by appropriate stimulation this sequence may be re- versed gives the heart a more diffuse character than the small intestine shows. But the uncertainty as to the myogenic or neurogenic na- ture of its beat leaves the heart a less clear example of these peculiarities than some other instances. As diffuse transmission is characteristic of the nerve-net and polarized transmission is a feature of the synaptic system, it fol- lows that the first signs of polarization in the nerve- net may be regarded as the initial step in the process of converting this organ into one of a synaptic type. As this process is apparently accomplished by the elongation of the neurofibril constitu- ents of the net in a particular direction, this feature may be regarded as the anatomical indication of the coming change. With the growth of such a feature the nerve-net begins to lose its diffuse condition and its transmitting fibrils come to form bands or trunks with some resem- blance to nerves; in other words, the nerve-net loses some of its net-like character and comes to develop a partial
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