50 TOPOGRAPHICAL ANATOMY OF THE 



examination of these circular bundles might lead to the supposition 

 that they are limited to one ventricle. It is stated, however, that they 

 circle round one ventricle, traverse the interventricular septum, and 

 then circle round the other ventricle. The wall of the left ventricle 

 contains also deep bundles in the neighbourhood of the left atrio- 

 ventricular fibrous ring, to which they are attached. 



Until comparatively recent years it was generally believed that 

 there was no connection between the musculature of the atria and that 

 of the ventricles. It has been shown, however, that a fasciculus of 

 somewhat modified cai'diac muscle, known as the atrio -ventricular 

 bundle, begins in a plexiform mass (the node of Tawara) on the right 

 face of the interatrial septum. The bundle gains the upper margin of 

 the septum between the ventricles and there bifurcates, one branch 

 passing to the left of the septum, and the other, the larger and more 

 definite, to the right. The right branch crosses the ventricle in the 

 transverse muscle (moderator band) and thus reaches the lateral 

 papillary muscle of this cavity. Another plexiform mass (the node of 

 Keith and Flack) with which the atrio-ventricular bundle is also 

 connected, is described as occurring close to the entrance of the cranial 

 vena cava into the atrium. 



At the junction of the atria and the ventricles, and round the 

 origins of the aorta and pulmonary artery, are rings of fibrous tissue in 

 association with which one or possibly two irregular masses of cartilage 

 may be found. The larger and more constant of these, lying cranial to 

 the termination of the caudal vena cava, is connected with the fibrous 

 rino- that surrounds the origin of the aorta, and to it is attached the 

 right caudal aortic semilunar valve. In old animals this cartilage may 

 be partially or completely ossified. A second, but smaller, cartilage 

 may be present at the origin of the aorta, and to it, when present, the 

 left caudal semilunar valve is attached. 



As has been previously remarked, the fibrous rings about the origin 



of the aorta and the pulmonary artery present crescentic notches into 



which the convex edges of the semilunar valves are received. 



The visceral portion of the serous pericardium, the epicardium, covers 



the heart externally. Between it and the myocardium, especially in 



the region of the grooves of the heart, is an accumulation of fat in 



greater or less abundance. 



The interior of the heart is lined by a smooth, shining membrane, 



the endocardium, continuous with the lining of the blood vessels. 



The cardiac valves may be regarded as strengthened duplicatures of the 



endocardium. 



Arrangement of the cardiac orifices. — Removal of the atria 



