'CONSTRUCTION OF THE HEART. 257 



the basis of attachment for the muscle bundles of the walls of 

 both the ventricles and auricles. 



In the ventricles many layers of muscles can be made out. The 

 outer fibres pass in a twisted manner from the base towards the 

 apex, where they are tucked in so as to reach the inner surface 

 of the ventricular cavity. They then pass back to be attached 

 at the base ; some passing into the papillary muscles are con- 

 nected with the cardiac valves through the medium of the chordae 



FIG. 115. 



Striated Muscle Tissue of the Heart, showing the trellis- work formed by the 

 short branching cells, with central nuclei. 



tendinese ; and the others, forming irregular masses of muscle on 

 the inner surface of the cavity, pass in various directions towards 

 the base, to be fused with the tendinous rings around the arterial 

 orifices. Another set of layers passes transversely around the 

 ventricle, lying between the inner and outer sets, and passing 

 nearly at right angles to them. 



The muscle fibres forming the thin auricular walls have their 

 origin from the zones of the auriculo-ventricular orifices, and pass 

 very irregularly around the cavities. The outer set of fibres 



22 



