36 CIRCULATION OF THE BLOOD ACTION OF THE HEAET. 



the columnae carneae. These fibres envelop both ventricles. 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 circu- 

 lar, 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 ordinary voluntary muscles ; but as already 



intimated, it presents certain pe- 

 culiarities in its minute anatomy. 

 The fibres are considerably small- 

 er and more granular than those 

 of ordinary muscles. They are, 

 moreover, connected with each 

 other by short, inosculating 

 branches. (See Fig. 17.) The 

 muscular fibres of the heart have 

 no sarcolemma. These peculiari- 

 ties, particularly the inosculation 

 of the fibres, favor the contrac- 

 tion of the ventricular walls in 

 every direction and the complete 

 expulsion 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-termi- 

 nations in its substance will be 

 described in connection with the 

 influence of the nervous system 

 upon the circulation. 



*cular fibres of Me ventricles (Bonamy and Each ventricle hag two ori . 



fices ; one b 



left ventricle* 6 f the heart ' 4 ' fibres penetrafcill s the 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 general 

 system. All of these openings are provided with valves, which are so ar- 

 ranged 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 sur- 

 face 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 ventri- 

 cle is completely filled and the fibres contract, they are forced up, their free 

 edges become applied to each other, and the opening is closed. 



