36 THE HEART 



a fibre it may only go part of the way across, or it may go 

 across in step-like fashion. 



According to v. Ebner these lines represent a dying phenomenon 

 a thickening of the fibre due to an abnormal contraction 

 during death. Eppinger regards them as pathological in origin. 

 M. Heidenhain, on the other hand, believes that they are really 

 present in the normal fibre, and have a special function to per- 

 form in regulating the growth of the fibre. He therefore terms 

 them " Schaltstiicke " (" regulators "). In the human heart they 

 are 1-1'7 /* thick, and consist of separate parallel perpendicular 

 rods. As we have said, they do not necessarily go right across a 

 fibre. The intervals at which they occur are by no means regular : 

 the pieces shut off by them may be of any length, and cannot be 

 looked upon in any way as cells. 



To sum up, the heart muscle, according to the more recent 

 investigations, is to be regarded as a syncytium in which a common 

 network of fibres has been developed. 



B. THE MORPHOLOGY OF THE VERTEBRATE HEART 



The study of the comparative anatomy of the vertebrate heart 

 greatly facilitates a proper understanding of the most recent 

 anatomical work on the mammalian heart namely, the isolation 

 of the auriculo-ventricular bundle. It aids us also in estimating 

 the significance of the many facts known about the heart, and 

 also in arriving at the probable value of the theories held in 

 regard to its working. Furthermore, the study of the comparative 

 anatomy is undoubtedly very helpful in bringing out points about 

 which research is still required and also in determining the lines 

 of such research. 



The Primitive Vertebrate Heart. Figure 1 is a generalised 

 diagram of such a heart. It consists of five chambers (a) The sinus 

 venosus ; (6) the auricular canal ; (c) the auricle ; (d) the ventricle ; 

 (e) the bulbus cordis. These parts are all in free muscular con- 

 tinuity ; there is no break at any of the junctional lines (1.1, 2.2, 

 4.4, 5, in Fig. 1). 



This is a point of interest, since in tracing the representatives 

 of these primary divisions in the mammalian, and especially in 

 the human, heart, it will be of service to ascertain whether this 

 muscular continuity between the different chambers still exists. 



