7G WARFARE IN THE HUMAN BODY 



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 ? 

 No new muscle fibres arise after birth, and yet 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 length, and are from two to 

 five times as broad. So far as we know there cannot be 

 new fibres in it. But in evolution new fibres are un- 

 doubtedly 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 in- 

 dividual 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. 



Without attempting a task of which I am incapable and 

 endeavouring to elucidate the problem of the origin of 

 circulatory systems in a primary vascular sponge-work, 

 through which the fluids of the primitive organism were 

 propelled by contractile tissue, it may be noted that the 

 columnce and chordce rise from such a sponge-work which, 

 at an early embryonic period, fills the primitive ventricle. 



