530 BLOOD-VASCULAR SYSTEM OF THE HORSE 



posterior vena cava; the orifice is provided witii a small semilunar valve or valve 

 of Thebesius (Valvula sinus coronarii). The middle coronary vein has a separate 

 opening in some cases close to that of the coronary sinus. Tlie atrio-ventricular 

 opening is in the lower part, and leads into the right ventricle. In addition to 

 the foregoing there are several small orifices of the vense cordis parvse; these 

 are concealed in the depressions l)et\veen the musculi pectin at i. 



The atrium is lined with a glistening memljrane, the endocardium. Its walls 

 are smooth except on the right and in the auricle (or appendix), where it is crossed 

 in various directions by muscular ridges, the musculi pectinati. Small liands 

 extend across some of the spaces inclosed by the nuisculi i)('ctinati. The latter 

 terminate above on a curved crest, the crista terminales, which indicates the 

 junction of the primitive sinus venosus of the eml)ryo with the atrium proper, 

 and corresponds with the sulcus terminalis externally'. The openings of the vense 

 cavse are without valves. A ridge, the intervenous crest, ^ projects downward and 

 forward from the roof just in front of the opening of the ]:osterior vena cava; it 

 tends to direct the flow of blood from the anterior vena cava to the atrio-ventricular 

 opening. The fossa ovalis is an oval depression in the septal wall at the point of 

 entrance of the i)osterior vena cava, bounded internally by a thick margin (Limbus 

 fossae ovalis). The fossa is the remains of an opening, the foramen ovale, through 

 which the two atria communicate in the fa?tus. 



The Left Atrium 

 The left atrium (Atrium sinistrian) or auricle forms the posterior part of the 

 base of the heart. It lies behind the pulmonary artery and the aorta and above 

 the left ventricle. The auricle (or appendix) extends outward and forward on the 

 left side, and its ]:)lind end is behind the origin of the pulmonary artery. The 

 pulmonary veins, usually seven or eight in number, open into the atrium behind 

 and on the right side. The cavity of the atrium is smooth, Avith the excej^jtion of 

 the auricle (or appendix), in which the musculi pectinati are present. In some 

 cases there is a depression on the septal wall opposite the fossa ovalis, bounded 

 above by a fold which is the remnant of the valve of the foramen ovale of the fcetus. 

 The atrio-ventricular opening is situated below and in front; it usually appears 

 smaller than the right one on account of the contraction of the left ventricle in the 

 dead subject. The apertures of small veins of the heart are found in the spaces 

 inclosed by the musculi pectinati. 



The nuinljer and the arrangement of the pulmonary veins are variable. They may l)e 

 five to nine in number. The largest orifice is posterior. I'sually three veins of considerable size 

 enter close together on the right above the posterior vena cava, and tln-ee or four open close 

 to the ridge which projects from the roof at the base of the auricle (appendix). 



The Right Ventricle 

 The right ventricle (Ventriculus dexter) constitutes the right-anterior part of 

 the ventricular mass. It forms almost all of the anterior border of the heart, but 

 does not reach the apex, which is formed entirely by the left ventricle. It extends 

 from the third rib to the fourth intercostal space on the left side, to the fifth ril) 

 and space on the right side. It is somewhat triangular in outline, and is semilunar 

 in cross-section. Its base faces upward and a little to the right and is connected 

 largely with the right atrium, with which it communicates through the atrio- 

 ventricular orifice; but its left part projects higher and forms the conus arteriosus, 

 from which the pulmonary artery arises. Its apex is two inches or more (ca. 5 to 

 6 cm.) above the apex of the heart. On opcniing the cavity it is seen that the two 

 openings are separated ])y a thick rounded ridge (Crista supraventricularis). 



^ This is termed the tubercle of Lower in human anatomy and the tuberculum intervenosum 

 by German veterinary anatomists. 



