74 WARFARE IN THE HUMAN BODY 



as much an organized dilatation sac of the whole fused 

 circulatory canal as the cured aneurism is of a part of it ? 

 It is in embryology that we seek for confirmation of what 

 is suggested by anatomy. But even anatomy alone offers 

 powerful proof of the view that the heart, as we know it, 

 is the latest result of repeated failures of the circulatory 

 canal under strain, and of the repairs effected by the stressed 

 tissues in their response to changed and abnormal stimuli, 

 just as bone alters under its particular stresses. During 

 embryological life there is found in the heart a small patch 

 of non-functioning muscle in the anterior segment of the 

 mitral valve. Its presence is intelligible if we consider 

 it a relic of a disrupted and repaired organ. The muscles 

 of the heart are obviously homologous with those of the 

 arteries. Yet they have become striated although they are, 

 of course, still involuntary. Non-striated muscle is the 

 earliest in evolution. It seems that the increased func- 

 tioning of the cardiac muscle has converted it into its 

 striated form, so that it resembles skeletal muscles, which 

 are much more active than non-striated muscle. The 

 whole histology of cardiac muscle probably represents the 

 result of great strains. Structures such as the disks or 

 bands of Ebarth are found nowhere else, and may be the 

 result of peculiar stress. There are even portions of muscle 

 which no longer perform muscular functions. Their 

 fibres do not contract, but serve instead to conduct 

 stimuli as if they were nervous tissue. All tissue is con- 

 ductive, but the Bundle of His, with its Purkinje fibres, 

 which carries the impulse from the auricle to the ventricle, 

 transmits messages at ten or twelve times the normal 

 muscular rate. When it fails there is heart-block. In 

 the embryo the valves arise from the cardiac walls, and are 

 composed of muscular tissue, which by the action of fibro. 



