386 



VETERINARY PHYSIOLOGY 



The muscular fibres of the auricles are arranged in two 

 badly-defined layers — 



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



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

 nected with the auriculo- ventricular rings. The inmost fibres 

 are raised into longitudinal ridges — -the pectinate muscles. 



Contraction of the first layer diminishes the capacity of 

 the auricles from side to side. Contraction of the second 

 set with the pectinate muscles draws the ventricles upwards 

 towards the auricles, and thus over the blood .that is beincf 



Fig. 164. — Cross Section through the Ventricles of the Heart looking towards 

 Auricles, to show the right Ventricle placed on the Central Core of the 

 left Ventricle. The Cusps of the Auriculo- ventricular Valves are also 

 shown. 



expelled from the auricles, and also pulls the auricles down- 

 wards 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 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. 164). 



