PATHOLOGICAL STATES IN EVOLUTION. 



245 



analogous, or shall I say homologous, results in the hypertrophied 

 heart. The normal male heart weighs about eleven ounces. In 

 some cases of aortic stenosis it may weigh over thirty ounces. 

 In such hypertrophied muscle are often found fibrous tissues 

 which probably represent the connective tissue of muscular fibres 

 which have atrophied from overstrain. The chordae tendinece of 

 the mitral valve are less muscular and more fibrous than the 

 same attachments of the tricuspid. This adaptation difference 

 lessens strain on the thinner right ventricle. It has, indeed, 

 remained thinner on that account. In the reptile with a function- 

 ing foramen the valves are purely mechanical, as pressure is 

 relieved by the patent orifice. The fossa ovalis in the mammal 

 is a remnant of the early communication between the auricles. 

 In a large number of normal hearts there is a small valvular 

 passage yet remaining in the left margin of the fossa. None of 

 these phenomena seem capable of explanation as the result of 

 spontaneous variations arising from some theoretic instability of 

 the organism. To argue that they are is to give biologic mystics 

 a chance. It appears obvious from all these facts taken together 

 that cardiac evolution has been a series of caused variations due 

 to increased and varying stresses which acted not only as a 

 moulding force on the shape and musculature of the heart but 

 on all its appendages. In the muscle of the ventricular walls 

 with its extraordinary complexity of layers and interlaced fibres 

 lies powerful evidence of such reactions. In both ventricles 

 there are seven muscular layers, while in the arteries there 

 seems but one. In the left ventricle these layers are obviously 

 thicker and stronger than in the less stressed right cavity. But 

 how did the ventricular cavities acquire more layers than the 

 arteries ? There is obvious reason for believing that stress can 

 be responded to by increase of muscle fibre during evolution. 

 In the gravid uterus the smooth fibres of the wall increase to 

 eleven times their normal lengih and are from two to five times 

 as broad. There may be new fibres in it. I doubt if any one 

 knows. But in evolution new fibres are undoubtedly found. In 

 the arteries, the fibres of non-striated muscle in the tunica media 

 are for the most part circular, but they appear to have more or 

 less longitudinal branches which interlock with like branches of 

 the neighbouring fibres. One of the most prominent features of 

 an individual aneurism is the thinning out, and sometimes the 

 disappearance, of the tunica media. The muscle fibres in such 

 cases are completely broken down, and if the aneurism is repaired 

 in individuals the work is done mostly by an increase of the 

 connective-tissue elements. The process is said by some to be a 

 reparatory endarteritis, in which the tissues of the adventitia 

 proliferate actively. But the evolutionary process has obviously 

 taken the path of increase and reactive proliferation of the 

 muscular elements of the media. 



It is often observed that the aneurism which displays sufficient 

 reacting power to the stresses of the blood stream accumulates 



Proc. Zool. Soc— 1918, No. XVIII. 18 



