534 BLOOD-VASCULAR SYSTEM OF THE HORSE 



compound. The moderator bands are variable. Commonly two or three larger 

 ones (which are often branched) extend from the musculi papillares to the septum. 

 Smaller ones may be found in various places, especially at the apex. The other 

 trabecuhe are fewer and less prominent than in the right ventricle. 



The interventricular septum (Septum ventriculorum) is the musculo-membran- 

 ous partition which separates the cavities of the two ventricles. It is placed ob- 

 liquely. One surface is convex, faces forward and to the right, and bulges into 

 the right ventricle. The other surface, which faces into the left ventricle, is con- 

 cave and looks backward and to the left. The greater part of the septum is thick 

 and muscular (Septum musculare), but the upper part is thin and membranous 

 (Septum membranaceum). The latter intervenes between the left ventricle, on 

 the one hand, and the right ventricle and atrium on the other. 



Structure of the Heart 



The heart-wall consists mainly of peculiar striped muscle, the myocardium, 

 which is covered externally by the visceral part of the serous pericardium or 

 epicardium, and is lined by the endocardium. 



The epicardium is in general closely attached to the muscular wall, but is 

 loosely attached over the coronary vessels and the associated subepicardial fat. 

 It consists of a layer of flat polygonal cells, resting on a membrane of white and 

 elastic fibers. 



The myocardium consists of planes of fibers arranged in a somewhat compli- 

 cated manner. The muscular tissue of the atria is almost completely separated 

 from that of the ventricles by the fil)rous rings around the atrio-ventricular orifices. 



In the atria the muscle bands fall naturally into two groups — superficial and 

 dec]). The former arc common to l^oth atria, the latter special to each. The 

 superficial or common fibers for the most part begin and end at the atrio-ventricular 

 rings, but some enter the interatrial septum. The deep or special bundles also 

 form two sets. Looped fibers pass over the atria from ring to ring, while annular 

 or spiral fibers surround the ends of the veins which open into the atria, the auricles, 

 and the fossa ovalis. 



The muscular wall of the ventricles is much stronger than that of the atria. 

 That of the left ventricle is in general about three times as thick as that of the right 

 one, but is thin at the apex. The superficial fibers are attached above to the atrio- 

 ventricular filjrous rings and pass in a s])ii'al toward the apex. Here they bend 

 upon themselves and pass deeply upward to terminate in a papillary muscle of the 

 ventricle opposite to that in which thej^ arose. The loops so formed at the apex 

 constitute a whorl, the vortex cordis. The deep fibers, although they appear to 

 be proper to each ventricle, have been shown by MacCallum to l)e in reality almost 

 all common to l)oth. Their arrangement is scroll-like. They begin on one side, 

 curve around in the wall of that ventricle, then pass in the septum to the opposite 

 side, and curve around the other ventricle. There is a layer of deep fibers which is 

 confined to the basal part of the left ventricle; it is attached to the left atrio- 

 ventricular ring. 



P'our fibrous rings (Annuli fibrosi) surround the orifices at the bases of the 

 ventricles. The atrio-ventricular rings separate the musculature of the atria from 

 that of the ventricles. Thos(> which surround the origins of the pulmonary artery 

 and aorta are festooned in conformity with th(^ attached borders of the valves. 

 The aortic ring contains on the right side a plate of cartilage (C'artilago cordis), 

 which frequently becomes more or less calcified in old animals. Sometimes a 

 smaller plate is present on the left side. 



The endocardium lines the cavities of the heart and is continuous with the 

 internal coat of the vessels which enter and leave the organ. Its free surface is 



