6 THE MECHANISM OF THE CIRCULATION. 



papillary muscles. Above this region there lies a space, which in the ox's 

 heart forms a very irregular cone pointing downwards. 1 The heart, therefore, 

 never empties itself completely in systole. 



The outer muscular layers 'of the heart take an oblique direction from right 

 to left, the inner layers take an oblique direction from left to right, and 

 the intermediate fibres run more or less transversely. Ludwig has shown that 

 in every piece of the walls of either chamber the fibres on the inner surface 

 take a direction across those on the outer surface, the intermediate fibres 

 taking, in regular order, a corresponding change of direction. 



The various fibrous structures of the base are all associated together, 

 forming one common fibrous membrane. This surrounds the aortic, the mitral, 

 and the tricuspid orifices, and, resting upon the base of the ventricles, gives a 

 general insertion to the muscular fibres of both of these cavities, and a general 

 origin to the muscular fibres of both auricles. 



In the dog the heart within its pericardium is slung above and below 

 by fibrous cords, which allow considerable displacement of the organ. 

 In man the pericardium is suspended above by the stout cords of the 

 deep cervical fascia ; below it is attached to the central tendon of the 

 diaphragm, in such a way as to form a triangular box of considerable 

 strength and rigidity. In this box the heart is slung. Almost all 

 lateral movement of the heart is thereby prevented, and since the 

 central tendon of the diaphragm is a fixed point, 2 the heart likewise 

 cannot descend. This fixation of the pericardium is a striking character- 

 istic of the human body, and its evolution in man can be traced to the 

 assumption of the erect posture. 



By means of a special mechanical contrivance (Fig. 27, p. 39), .Roy 

 and Adami 3 have measured the approximation of any two given points 

 on the exposed heart during systole. They determined that if two 

 lines be drawn parallel either to the intraventricular furrow or the 

 auriculo-ventricular groove, these lines approximate in systole. There- 

 fore the exposed heart shortens both in its longitudinal and transverse 

 diameters. 



It is agreed by nearly all observers that the apex does not move 

 up to the base, but the base moves down towards the apex. The 

 latter remains practically a fixed point. The explanation of this 

 fact is partly to be found in the recoil of the heart. When the blood 

 is shot out into the aorta, the heart " kicks back " like a gun on its 

 discharge. The momentum of recoil equals the momentum of output. 

 Thus the heart is jerked downwards with considerable force, and 

 at the dead point there must be a slight pause before it returns. 

 If the veins which feed the heart be ligatured, this recoil fails 

 to take place, and the base does not move towards the apex. 4 



The recoil is probably not the whole explanation of the descent 

 of the base. The aorta and pulmonary artery as they leave the heart 

 form a spiral figure ; the blood thrown into them on systole not only 

 extends these arteries but partly undoes the spiral. This may contribute 

 to the descent of the base. 



The middle muscular layers of the heart are called by Krehl 5 the 



1 Sandborg and Worm Midler, Arch. f. d. ges. Physiol., Bonn, 1880, Bd. xxii. S. 424. 



2 Groiiraea, Anat. Anz., Jena, 1897, Bd. xiii. S. 562. The evidence is furnished by 

 Rbntgen ray photographs. 



3 Practitioner, London, 1890, vol. i. p. 82. 



4 Guttmann", Firchow's Archio, 1875, Bd. lxv. S. 540. 



6 Abhaiull. d. math.-phys. CI. d. k. siichs. Gcscllsch. d. Wisscnsch., 1891, Bd. xvii. S. 346. 



