THORACIC CAVITY 93 



with the fibrous atrio-ventricular rings, and they pass over the auricles 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. 74^ It has been assumed that the impulses which regulated the 

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

 the fibres of this 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 

 inferior vena cava, and the coronary sinus ; and into the left atrium through 

 the four pulmonary veins. The atrial contraction commences with the 

 contraction of the circular fibres which surround the mouths of the veins 

 entering the atria, and thus the blood is prevented from passing back 

 into the veins. As the contraction extends to the general fibres of the atria 

 the blood is forced onwards into the ventricles, which become distended. 

 Then the ventricular contraction commences, the atrio-ventricular valves 

 close, and, as the contraction proceeds, the blood is driven out of the 

 ventricles through the arterial orifices, that in the right ventricle being 

 ejected into the pulmonary artery, and that in the left ventricle into the aorta. 



When the ventricular contraction is completed the period of diastole 

 commences ; and, as long as the heart remains alive, the cycle is repeated. 



The work of the atria is merely to force the blood through the widely 

 open atrio-ventricular orifices into the ventricles and to expand the dilating 

 walls of the ventricles. For this purpose no great force is required, there- 

 fore the walls of the atria are thin. The work of the ventricles is much 

 more severe, therefore their walls are thicker, but the right ventricle has 

 only to exert sufficient force to drive the blood through the lungs to the left 



