988 A MANUAL OF ANATOMY 



minute veins are said to open into the left auricle, and also into 

 both ventricles. 



Lymphatics.— The lymphatic vessels of the heart form two net- 

 works — subendocardial and subpericardial. The vessels are ulti- 

 mately collected into two trunks, right or posterior, and left or 

 anterior. The right trunk receives the lymphatics of the right side 

 of the heart, and the left trunk takes up those of the left side. 

 Each trunk accompanies the corresponding coronary artery, and 

 both pass backwards on either side of the common pulmonary 

 artery. Having pierced the pericardium, they terminate in the 

 superior mediastinal or cardiac glands. 



Nerves. — ^The nerves of the heart are derived from the super- 

 ficial and deep cardiac plexuses, and, through these, from the 

 pneumogastric and sympathetic nerves. The branches to the 

 auricles are derived partly from the deep cardiac plexus, and 

 partly from the coronary plexuses. They form a gangliated 

 plexus on the surface of each auricle beneath the epicardium, and 

 from this plexus branches proceed to the muscular wall. The 

 branches to the ventricles are derived from the right and left cor- 

 onary plexuses. These branches lie upon the surface of each 

 ventricle beneath the epicardium, and in the heart of the calf 

 they are readily recognisable as delicate, thread-like streaks. 

 Minute ganglia are met with, at intervals, on these nerves in the 

 region of the base of the ventricles, but none on those nerves which 

 lie over the lower two-thirds of each ventricle. The branches 

 which enter the muscular, substance of the ventricular walls form 

 plexuses, but are destitute of ganglia. 



Structure of the Heart. — The cardiac wall is composed of muscular tissue, 

 which is known as the myocardium. This is intimately covered by the 

 visceral layer of the serous portion of the pericardium, this layer being called 

 the epicardium, and the cavities of the heart are lined with a membrane 

 which is known as the endocardium. 



The epicardium resembles a serous membrane in structure, and is covered 

 by endothelium. It is provided with stomata, which establish a communica- 

 tion between its lymph-canalicular system and the cavity of the pericardium.^ 

 Beneath the epicardium there is a variable amount of fat, which is chiefij 

 met with in the auriculo-ventricular and interventricular grooves. 



The endocardium is a smooth, delicate membrane, destitute of blood-- 

 vessels, and covered on its internal surface by endothelium. It consists 

 of a connective-tissue basement membrane, containing elastic fibres, and is 

 continuous, through the arterial and venous orifices, with the tunica intitnc 

 of the vessels. It enters into the formation of the segments of the auriculo-' 

 ventricular and semilunar valves. 



Fibrous Tissue. — The auriculo-ventricular and arterial orifices of the heart 

 are each surrounded by a ring of fibrous tissue, and these rings, in the case of 

 the auricles and ventricles, connect them together. The fibrous tissue of 

 these rings furnishes that which is met with in the segments of the various 

 valves. In the triangular interval between the aortic and the two auriculo- 

 ventricular orifices there is a collection of dense fibrous tissue, of the con- 

 sistence of fibro-cartilage, which is connected with the fibrous rings just 

 referred to. When a heart is boiled, the fibrous tissue is dissolved, and the 

 auricular portion can be separated from the ventricular portion. 



Myocardium. — The muscular tissue of the auricles is mostly distinct from 



