628 BLOOD-VASCULAR SYSTEM OF THE HORSE 



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 fibrous rings around the atrio-ventricular orifices. 



The connection between the musculature of the atria and that of the ventricles is established 

 by the atrio-ventricular bundle (Fasciculus atrio-ventricularis) . This begins as a network of 

 fibers about the opening of the coronary sinus and the adjacent atrial wall. The fibers con- 

 verge to a flat, irregular mass at the upper border of the ventricular septum. From this two chief 

 divisions proceed. One of these descends on the right side of the ventricular septum and passes 

 by the moderator band to the lateral papillary muscle. The other branch descends on the left 

 side of the septum and ramifies on the wall of the ventricle. The left branch is somewhat diffi- 

 cult to f oUow, since it is thin and reticulate and is covered in great part by a layer of ventricular 

 muscle fibers. The right branch is subendocardial. The bundle and its divisions are enclosed 

 in a fibrous sheath. The functional importance of the bundle in mediating the contraction 

 wave was shown by Erlanger, who found that clamping the bundle caused heart-block. 



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

 deep. The former are conmion to both 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 fibrous rings and pass in a spiral 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 they arose. The loops so formed at the apex 

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

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

 all common to both. 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. 



Four 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. Those which surround the origins of the pulmonary artery 

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

 The aortic ring contains on the right side a plate of cartilage (Cartilago 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 

 smooth and glistening and is formed by a layer of endothelial cells. The latter 

 rest on a thin layer of fibro-elastic tissue, which is connected with the myocardium 

 b}' a subendocardial elastic tissue containing vessels and nerves. 



Vessels and Nerves. — The heart receives a large blood-supply through the two 

 coronary arteries which arise from the aorta opposite to the anterior and left cusps 

 of the aortic valve. Most of the blood is returned by the coronary veins, which 

 open into the right atrium by the coronary sinus. ^ A few small veins open directly 

 into the right atrium, and others are said to open into the left atrium and the 

 ventricles. The Ijrmph-vessels form a subepicardial network which communicates 



1 These vessels will be described later in their systematic order. 



