212 HUMAN PHYSIOLOGY 



dissolves it, and it is by white fibrous tissue that the auricles 

 and great arteries are attached to the ventricles. This tissue 

 is arranged in three rings, one encircling the opening between 

 the right auricle and ventricle and crescentic in shape ; one, 

 more circular in shape, encircling in common the left auri- 

 culo-ventricular arid the aortic orifice, and one encircling 

 the pulmonary opening. The auricles are attached to the 

 auriculo-ventricular rings above, the ventricles are attached 

 below, while the valves of the heart are also connected with 

 them. I v 



The muscular fibres of the auricles are arranged in two 

 badly defined layers . 



1st. An outer layer runnings, horizontally round both 

 auricles. 



2nd. An inner layer arching over each auricle, and con- 

 nected with the auriculo-ventricular rings. 



Contraction of the first layer diminishes the capacity of 

 the auricles from side to side. Contraction of the second 

 pulls the auricles downwards towards the ventricles, and 

 thus diminishes their eapacity from above downwards. 



The peculiar striped muscle fibres of the auricular walL 

 extend for some distance, along the great veins which open 

 into these chambers. 



The left ventricle forms the cylindrical core to the heart, 

 and the right ventricle is attached along one side of it. The 

 septum between the ventricles is essentially the right wall of 

 the left ventricle, and it bulges into the right ventricle with 

 a double convexity from above downwards and from before 

 backwards (Fig. 106). 



The muscle fibres of the ventricle are arranged essentially 

 in three layers 



1st. The outmost layer takes origin from the auriculo- 

 ventricular rings, and passes downwards and to the left till 

 it reaches the apex of the heart. Here it turns inwards, 

 forming a sort of vortex, and becomes continuous with the 

 inmost layer. 



2nd. The middle layer is composed of fibres running hori- 

 zontally round each ventricle. It is the thickest layer of 

 the heart, and in contracting it pulls the walls of the ven- 

 tricles towards the septum ventriculi. 



