$2 A MANUAL OF VETERINARY PHYSIOLOGY 



runs downwards into the ventricular septum, and is distributed 

 to the musculi papillares and moderator bands. In the left 

 heart the bundle reaches the left ventricle from the right auricle ; 

 it then runs down the ventricular septum, and, like its fellow in 

 the opposite side of the heart, is distributed to the musculi 

 papillares and moderator bands. 



The muscular walls of the auricle and ventricle are constructed 

 of layers of red fibres, varying greatly in thickness and of ex- 

 tremely complex disposition, especially around the ventricular 

 cavities. The ventricular walls are thicker than those of the 

 auricles, and the left side of both cavities is better developed than 

 the right. In certain portions of the right auricle the wall is so 

 thin as to be semi-transparent, and appears, in fact, to consist of 

 little else than the two layers of serous membrane which cover 

 and line the heart. The ventricular walls are of uneven thickness ; 

 at the apex of the heart they are reduced to a few fibres of muscular 

 tissue one-eighth of an inch in thickness. Chauveau, in fact, says 

 that at this point there is nothing more than two layers of serous 

 membrane — viz., that lining and that covering the heart. 



The varying thickness of the walis of the heart is due to the 

 complex arrangement of the various layers of fibres ; these may, 

 broadly speaking, be divided into two main groups — an internal, 

 belonging to each auricle and each ventricle separately, and an 

 external group, belonging to both auricles and both ventricles. 

 Excluding the A. V. bundle just described, the fibres of the auricles 

 are confined to the auricles, and the fibres of the ventricles to the 

 ventricles ; the advantage gained by this arrangement is obviously 

 connected with the independent contraction of auricles and 

 ventricles. 



The muscular layer peculiar to each ventricle is very complex, 

 but, generally speaking, may be described as a scroll of fibres of 

 several layers running obliquely around each ventricle, and 

 where the scrolls meet forming the ventricular septum. The 

 scrolls are attached above to the auriculo- ventricular ring, but 

 are left open below. If we can imagine these scrolls separated 

 from the other muscle of the heart, they would present the appear- 

 ance of a pair of hollow cones. Covering this internal layer is an 

 external, belonging in this case to both ventricles ; it takes its 

 origin from the auriculo-ventricular ring, and describes a spiral 

 course in descending from base to apex of the heart, where the 

 fibres form what Henle described as a vortex (Fig. 13) ; they 

 then pass upwards through the opening in the scrolls of the inner 

 layer, and so gain the interior of the ventricles. Some of the 

 fibres pass to the columnce carnece, others to the musculi papillares, 

 while the majority gain insertion into the auriculo-ventricular 

 ring from which they had their origin. As pointed out by 



