ANIMAL MECHANICS. 



437 



vortex, from which point they return almost in a straight line 

 towards the tendinous zone from which they originally 

 started, but on the return path they now form the innermost 

 wall of the ventricle ; so that these fibres during their spiral 

 course are the outermost, and during their direct course are 

 the innermost fibres of the heart. 



The fibres which lie inside the superficial fibres, after de- 

 scribing a complete spiral, plunge into the interior of the 

 heart before reaching the vortex, and return back towards the 

 tendinous zone almost in a straight line, lying outside the 

 first group of fibres which line the cavity of the ventricle. 

 There is thus a sort of bag-shaped cavity formed round each 

 ventricle, between the outside of the heart and the walls of 

 the ventricle. 



These bag-shaped cavities are now to be supposed occu- 

 pied by the proper fibres of each ventricle. 1 was able to 

 trace the complete course of the proper fibres of the left ven- 

 tricle, filling up the space between the spiral and direct por- 

 tions of the common fibres, and I found that the proper fibres 

 wind in a spiral line round the ventricle, and return back 

 towards the tendinous zone, after describing a circumference 

 and one-fifth. 



Thus, it will be seen that the common and proper fibres 

 are similarly arranged, and may be divided each into two 

 portions ; the first portion spiral and outer, and the second or 

 return portion direct and inner. The common fibres, how- 

 ever, return to the same azimuth on the circumference of the 

 tendinous ring, from which they started, after having described 

 a complete circumference in their course ; while the proper 

 fibres return to an azimuth on the tendinous ring 7 2° in ad- 

 vance of the azimuth from which they started, after having 

 described a complete circumference and one-fifth in their 

 course. 



It is easy to see that as the common fibres describe a 



