596 CIRCULATORY APPARATUS. 



the free course of the blood. The extremely fine and close connective tissue 

 uniting the muscular fibres, and the anastomosing of these, is another peculiarity 

 of structure that must be favourable to the solidarity of their action, which 

 ought to be sinniltaneous. In the healthy organ, adipose tissue is only found ir 

 the grooves on its surface, around the vessels lodged in them, and particularly at 

 its base, between the large arterial trunks.) 



C. Vessels and Nerves of the Heart. — Blood is carried to the muscular 

 tissue of the heart by two large vessels — the coronarij arteries. They emanate 

 from the trunk of the aorta, at the sigmoid valves, and each divides into two 

 principal branches — one passing along the horizontal, the other in the vertical 

 furrow of the heart. Collectively, these arteries form two circles, which surround 

 the heart in intersecting it at a right angle in the auriculo-ventricular groove. 



The blood is carried from the walls of the heart by a single but important 

 veiii, which empties itself into the right auricle. 



The lymphatics follow the arteries, passing aiong the visceral layer of the 

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

 heart. (The epicardium, as well as the endocardium — especially in the ventricles — 

 has a large network of fine lymphatics, the walls of which consist of only a 

 single layer of intimately adhering cells. Lymphatics are also numerous in the 

 myocardium, and amongst the muscular fibres there are lacunse or spaces lined 

 by endothelial cells — the origins of the lymphatics. The lymph passes into 

 lymphatic glands lying between the aorta and trachea, flowing thence into the 

 thoracic duct. No lymphatic vessels have been traced upon the chordse tendinese, 

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



The nerves of the heart, furnished by the cardiac plexus, <;ome from the 

 pneumogastric 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, Bidder, and Ludwig have called attention. 

 It is believed that there exist three ganglia in different points of the cardiac 

 parietes, and that on these depend the movements of the organ. (Nerve-cells are 

 most numerous in the basal and middle parts of the ventricles of the heart of the 

 Dog, Sheep, Calf, and Pig, chiefly in the anterior and posterior inter-ventricular 

 grooves and on the left ventricle. 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. 4. Fibres 

 coursing in the vagus — nervi cardiaci — and originating in a centre situated in 

 the medulla oblongata. The first three of these ganglia and fibres probably 

 collectively constitute the excito-motor system of the heart ; the fourth is an 

 inhibitory, restraining, or regulo-motor centre.) 



D. Serous 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 — epicardial — a dependency of the fibro-serous sac which contains 

 the heart. 



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

 dent, like the cavities they line, are spread over the auricular and ventricular 

 walls, covering the tendinous or muscular columns attached to these walls, and 



