THORACIC CAVITY 121 



cardium are disposed in layers, in each of which the fibres take a special 

 direction. 



The arrangement of the various layers of the myocardium cannot be 

 displayed in an ordinary dissecting-room heart, in which the continuity of 

 the fibres has been destroyed by the incisions made to display the cavities, 

 but the arrangement of the layers is practically the same in the hearts of 

 all mammals. Therefore, for the purpose of studying the layers, the 

 dissectors should obtain a sheep's heart. Fill the heart with a paste 

 made of flour and water ; then boil it for a quarter of an hour. The 

 boiling expands the paste, softens the connective tissue, and hardens the 

 muscular fibres. After the boiling is finished the heart should be placed 

 for a time in cold water. After it has cooled, first the epicardium and then 

 the muscular fibres should be gradually torn off. 



The atrial fibres are difficult to dissect. They consist of three groups : 

 (i) A superficial group, running more or less transversely and common to 

 both atria. They are best marked near the coronary sulcus. (2) A deep 

 group, special to each atrium. The extremities of the fibres of the deep 

 group are connected with the fibrous atrio-ventricular rings, and they pass 

 over the atria from front to back. (3) The third group consists of sets 

 of annular fibres surrounding the orifices of the veins which open into the 

 atria. 



The fibres of the ventricles are more easily dissected. They consist, 

 for the main part, of two groups the superficial and the deep. The fibres 

 of each set are common to both ventricles, and the dissectors should note 

 the remarkable spiral or whorled arrangement of the superficial fibres 

 which occurs at the apex, where they pass into the deeper parts of the wall. 



The superficial fibres spring mainly from the fibrous atrio-ventricular 

 rings. Those which are attached to the right ring turn inwards at the 

 apex and become continuous with the papillary muscles of the left ventricle, 

 whilst the fibres which spring from the left ring pass in the same way to 

 the papillary muscles of the right ventricle. The deeper fibres form an 

 CO-shaped layer, one loop of the CO surrounding the right and the other the 

 left ventricle. 



The fibrous rings of the atrio-ventricular orifices intervene between the 

 atrial and the ventricular muscle fibres, but the two groups are brought 

 into association with each other by the atrio-ventricular bundle described 

 on p. 101. It has been assumed that the impulses which regulate the 

 movements of the ventricles were conveyed to them from the atria by 

 the fibres of that bundle, but it has been shown recently that numerous 

 nerve fibrils are intimately intermingled with the fibres of the atrio- 

 ventricular bundle. It is possible, therefore, that the connection between 

 the atria and the ventricles is neuro-muscular. 



The Action of the Heart. The differences between the various parts of 

 the heart, i.e. the thinness of the walls of the atria as contrasted with the 

 thickness of the walls of the ventricles, and the greater thickness of the 

 walls of the left as contrasted with those of the right ventricle, are associated 

 with the functions of the various chambers, and with the action which the 

 heart plays in the maintenance of the circulation of the blood. The heart 

 is a muscular pump, provided with receiving and ejecting chambers. It 

 has three phases of action : (i) a period of atrial contraction ; (2) a period 

 of ventricular contraction, which immediately succeeds the atrial con- 

 traction ; (3) a period of diastole or rest. 



During the period of diastole or rest the chambers, previously con- 

 tracted, dilate as the muscular fibres of the heart relax. The dilatation is 

 aided by the respiratory movements of the thorax. As the dilatation pro- 

 gresses blood flows into the right atrium from the superior vena cava, the 



