54 THE ELEMENTARY NERVOUS SYSTEM 



that of recent years the myogenic theory, which took its 

 origin in the very early work of Harvey, has on the whole 

 gained precedence (Lewis, 1917) and that this is chiefly 

 due to the work of Gaskell, Engelmann, and their fol- 

 lowers. According to these investigators the heart in the 

 lower vertebrates is a muscular tube bent in the form of 

 a letter S and dilated into a series of four chambers (Fig. 

 13). The heart becomes complicated in the higher forms 



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 





