. The elementary nervous system . Fid. 13.—Diagram of the heart of a fish showing in lateral view its four chambers, the venous sinus s, the auricle a, the ventricle u and the bulb 6. The s-shaped axis is indicated by the arrow. chiefly through the appearance of a set of partitions whereby this organ is divided into right and left halves. Contraction normally begins in the muscle of the most posterior chamber, the venous sinus, or when this cham- ber is incorporated in the next chamber in advance, the auricle, in the muscle of the posterior portion of this chamber of the heart. The spot in whi
. The elementary nervous system . Fid. 13.—Diagram of the heart of a fish showing in lateral view its four chambers, the venous sinus s, the auricle a, the ventricle u and the bulb 6. The s-shaped axis is indicated by the arrow. chiefly through the appearance of a set of partitions whereby this organ is divided into right and left halves. Contraction normally begins in the muscle of the most posterior chamber, the venous sinus, or when this cham- ber is incorporated in the next chamber in advance, the auricle, in the muscle of the posterior portion of this chamber of the heart. The spot in which contraction originates is called in the hearts of the higher vertebrates the sino-auricular node. From this node the wave of contraction spreads over the auricle and across the nar- row bridge of muscle, to the ventricle. Here it is rapidly propagated by the specialized muscular tissue of the in- ner face of the ventricle, the so-called Purkinye tissue, over the whole of this part of the cardiac muscle, which is thus brought into a unified contraction. According to this view contraction arises in the muscle itself and is
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