THE HEART. 



511 



The lympliatics follow the arteries, passing along the visceral layer of 

 the pericardium, aud entering the cluster of glands situated near the base of 

 the heart. (The endocardium, especially in the ventricles, has a network 

 of fine lymphatics, the walls of which consist of only a single layer of 

 intimately-adhering cells. None have been traced upon the cliordes tendinere, 

 and very few upon the auriculo-ventricular and semilunar valves.) 



The nerves of the heart, furnished by the cardiac plexus, come from the 

 pneuniogastric and sympathetic. The tubes are small, and show some cells 

 in their course. In addition to these, the heart is provided with a particular 

 ganglionic system, to which Remak has called attention. It is believed tbat 

 there exist three ganglia in different points of the cardiac parietes, and that 

 on these depend the movements of the organ. (According to Carpenter, the 

 nerves of the heart are : 1, Minute ganglia and fibres of the sympathetic, 

 situated in the walls of the cavities, and especially in the auriculo- 

 ventricular furrow; 2, Fibres derived from the cervical portion of the 

 sympathetic, and passing to the cardiac plexus, between the aorta and 

 pulmonary artery ; 3, Cerebro-spinal fibres entering the inferior cervical or 

 stellate ganglion, and proceeding to the same plexus, and probably derived 

 from a centre situated in the brain and spinal cord ; and, 4, Fibres coursing 

 in the vagus, and originating in a centre situated in the medulla oblongata. 

 The first three of these ganglia and fibres probably collectively con- 

 stitute the excito-motor system of the heart, .the fourth is an inhibitory, 

 restraining, or regulo-motor centre.) 



D. Sekous Membranes of the Heart. — These are three in number : 

 two internal, or endocardial, one of which occupies the right, the other the 

 left cavity; and an external, a dependency of the fibro-serous sac which 

 contains the heart. 



1. Internal serous membranes, or endocardia. — These two membranes, 

 independent, like the cavities they line, are spread over the auricular and 

 ventricular walls, covering the tendinous or muscular columns attached to 

 these walls, and are prolonged into the veins and arte- 

 ries, to form the internal tunic of these vessels. At 

 the auriculo-ventricular and arterial openings, they con- 

 stitute a duplicature for the valves situated there. 

 These valves are, therefore, due to the projection of a 

 circular fold of the endocardia, between the two layers 

 of which a thin prolongation of the fibrous zones from 

 the base of the heart is insinuated. In the auriculo- 

 ventricular valves there is also found, beneath the 

 internal or superior layer, muscular fibres furnished by 

 the auricles. 



The endocardium of the right heart has a red tint, 

 which is deepest in the ventricle. In the left heart, 

 this tint is slightly yellow, especially in the walls of 

 the auricular appendix, which may be attributed to the 

 presence of a thin layer of yellow elastic tissue that 

 covers the adherent face of the membrane. 



(The endocardium consists of three layers: 1, A 

 thin bed of white fibrous tissue, connecting it to the 

 muscular structure; 2, A middle layer, composed of 

 elastic tissue which is very abundant in the auricles ; 

 and 3, An epithelium, consisting of a single or double 

 layer of somewhat elongated, polygonal, pavement nucleated cells.) 



Fig. 264. 



EPITHELIUM OF THE 

 ENDOCAEDIUM. 



1, Nucleated cells as- 

 suming the fusiform 

 figure ; 2, Polygonal 

 nucleated cells. 



