38 



CIRCULATION OF THE BLOOD. 



42 



These fibres envelop both Tentricles. They may be said to arise from cartilaginous rings 

 which surround the auriculo- ventricular orifices. The external surface of the heart is 

 marked by a little groove which indicates the division between the two ventricles. The 



deep fibres are circular, or transverse, 

 and surround each ventricle separately. 

 The muscular tissue of the heart 

 is of a deep-red color and resembles, in 

 its gross characters, the tissue of ordi- 

 nary voluntary muscles ; but, as already 

 intimated, it presents certain peculiar- 

 ities in its minute anatomy. The fibres 

 are considerably smaller and more gran- 

 ular than those of ordinary muscles. They 

 are, moreover, connected with each other 

 by short inosculating branches, while in 

 the voluntary muscles each fibre runs 

 from its origin to its insertion enveloped 

 in its proper sheath, or sarcolemma. The 

 muscular fibres of the heart have no sar- 

 colemma. These peculiarities, particular- 

 ly the inosculation of the fibres, favor the 

 contraction of the ventricular walls in 

 every direction and the complete expul- 

 sion of the contents of the cavities with 

 each systole. 



The distribution of the nerves to the 

 heart and the arrangement of the ganglia 

 and nerve-terminations in its substance 

 will be taken up in connection with the 

 influence of the nervous system upon he 

 action of the heart. 



Each ventricle has two orifices; one 

 by which it receives the blood from the 

 auricle, and the other by which the blood 

 passes from the right side to the lungs 

 and from the left side to the system. 

 All of these openings are provided with 

 valves, which are so arranged as to allow the blood to pass in but one direction. 



Tricuspid Valve. This valve is situated at the right auriculo-ventricular opening. It 

 has three curtains, formed of a thin but resisting membrane, which are attached around 

 the opening. The free borders are attached to the chordae tendineae, some of which arise 

 from the papillae on the inner surface of the ventricle, and others, directly from the walls 

 of the ventricle. When the organ is empty, these curtains are applied to the walls of 

 the ventricle, leaving the auriculo-ventricular opening free ; but when the ventricle is 

 completely filled and the fibres contract, they are forced up, their free edges become 

 applied to each other, and the opening is closed. 



Pulmonic Valves. These valves, also called the semilunar or sigmoid valves of the 

 right side, are situated at the orifice of the pulmonary artery. They are strong mem- 

 branous pouches, with their convexities, when closed, looking toward the ventricle. 

 They are attached around the orifice of the pulmonary artery and are applied very 

 nearly to the walls of the vessel when the blood passes in from the ventricle ; but at 

 other times their free edges meet in the centre, opposing the regurgitation of blood. At 

 the centre of the free edge of each valve is a little corpuscle called the corpuscle of 



FIG. 14. Right cavities of the heart. (Bonamy and Beau.) 

 I, right ventricular cavity; 2, posterior curtain of the 

 tricuspid valve ; 3, right auricular cavity ; 4, colum- 

 nce camece of the right auricle ; 5, section of the coro- 

 nary vein ; 6, Eustachian valve ; 7, ring of Vieussens ; 

 8, fossa ovalis ; 9, superior vena cava ; 10, inferior -vena 

 cava ; 11, aorta ; 12, 12, right pulmonary veins. 



