THE HEART 



33 



Chauveau, whose account of the arrangement of the fibres in 

 the heart of the horse we have mainly followed, this layer forms 

 between its origin and insertion figure of eight loops, the smallest 

 loop being at the apex of the heart, where at its centre it leaves 

 a very small space, through which a probe may be passed into 

 the ventricles without piercing anything but the external and 

 internal layer of serous membrane. The external fibres of the 

 right ventricle are arranged much the same as those of the left, 

 but, according to Pettigrew, they do not pass into the ventricle 

 at a single point as in the left ventricle, but obtain entrance all 

 along the anterior coronary groove. 



The fibres of the auricle are much simpler in their arrangement. 

 Those peculiar to each cavity are disposed in several fasciculi, 

 some circular, especially surrounding the mouths of the vessels, 

 others in the general body of the auricle in interwoven loops. 

 The septum is formed where 

 the two sets proper to each 

 auricle meet. The fibres com- 

 mon to both auricles are gener- 

 ally arranged transversely. 



It may be added that 

 anatomists are not agreed as 

 to the arrangement of the fl 



fibres of the heart muscle, : fev\ - 



and this may to some extent wy ■•fy 



be due to the fact that the PH ~S 



system is not the same in all 



J . , Fig. 13. — Apex of Heart, showing 



animals. Vortex Arrangement of Fibres 



The arrangement just out- (after Krause). 



lined certainly appears to 



provide for the squeezing and wringing movement to which the 

 ventricular contents are exposed, the shortening of the heart 

 wall from base to apex, and the contraction of the musculi 

 papillares at the moment the valves close. Far simpler is the 

 disposition of the muscular fibres of the auricles ; physio- 

 logically these cavities may be regarded as the dilated extremity 

 of the vessels entering the heart, and their function is more that 

 of a well than a pump. Nevertheless, the existence of a network 

 of muscular pillars in the auricles, especially the dense bands 

 in the left, warn us to be careful not to regard these cavities in 

 the horse as mere passive channels of the circulation. 



The cavities of the heart are lined by the endocardium which 

 is reflected over the valves ; this membrane in the left auricle of 

 the horse is of a peculiar grey colour. 



Certain fibrous rings are found in the heart where the valves 

 are situated, to which these and the muscular fibres obtain a 



3 



