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 etrio-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 follow, 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 common 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 whorl, 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 
by 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 lymph-vessels form a subepicardial network which communicates 
1 These vessels will be described later in their systematic order. 
