228 VETERINAKY PHYSIOLOGY 



heart be boiled, the auricles, the aorta and the pulmonary 

 artery may be separated from the ventricles. This is because 

 boiling converts fibrous tissue to gelatine and 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 auriculo- 

 ventricular and the aortic orifice, and one encircling the 

 pulmonary opening. The auricles are attached to the auri- 

 culo ventricular rings above, the ventricles are attached below, 

 while the valves of the heart are also connected with 

 them. 



The muscular fibres of the auricles are arranged in two 

 badly-defined layers 



1st. An outer layer runs horizontally round both auricles. 



'2nd. An inner layer arches over each auricle, and is 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 capacity 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 (figs. 108 and 110). 



The muscle fibres of the ventricles 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 



O 



inmost layer. 



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

 zontally round each ventricle. It is the thickest layer of 



