306 



CIRCULATORY SYSTEM. 



The heart (Figs. 157, 158) is a hollow muscular organ whose 

 functions consist in acting as a reservoir and also as a pump, the 

 auricles being the reservoir and the ventricles being the pump 

 (Plate 2). It is about ,12.5 cm. long, 8 cm. wide in its widest 

 part, and 6.3 cm. thick at its thickest part ; its weight is about 

 300 grams in the adult. It has a conical form, its base being 

 above and to the right, and its apex below and to the left. It is 



divided longitudinally by a 

 partition or septum into a 

 right and a left half, which 

 are sometimes denominated 

 the right heart and the left 

 heart. Each half is com- 

 posed of an auricle and a 

 ventricle ; thus there are 

 four cavities the right au- 

 ricle, the right ventricle, the 

 left auricle, and the left ven- 

 tricle. 



The right auricle (Fig. 

 157) is somewhat larger than 

 the left, and has the thinnest 

 walls of the four cavities, 

 measuring about 2 mm. in 

 thickness. Discharging into 

 this cavity are the superior 

 and inferior venae cavaB, at the 

 mouths of which there are no 

 valves. Within the cavity is 



FIG. 157. Interior of right auricle and the Eustachian valve, which 



ventricle, exposed by the removal of a part W JH f ur ther be described 

 of their walls : 1, superior vena cava ; 2, in- , . . , , 



ferior vena cava ; 2', hepatic veins ; 3, 3', 3", when dlSCUSSing the fetal Cir- 



inner wall of right auricle; 4, 4, cavity of culation, This Valve IS sit- 



^r^'S^SSig-S ted between the opening 



tery, in the wall of which a window has of the inferior vena cava and 



been cut ; 7, on aorta near the ductus arte- f u anri VnlovpntnVnlir orifirp 



riosus; 8, 9, aorta and its branches; 10, 11, l ? V , ei 



left auricle and ventricle (Allen Thomson). The right ventricle (1 lg. 



1 57) has walls whose thickness 



is greater than that of either auricle, but less than that of the left 

 ventricle. The cavity of the right ventricle communicates with 

 that of the right auricle by the right auriculoventricular orifice, 

 at which is situated the tricuspid valve. It ordinarily contains, 

 when filled, 87 grams of blood (p. 314). Connected with this 

 ventricle is the pulmonary artery, at whose point of junction w r ith 

 the ventricle is the pulmonary orifice, at which is situated the pul- 

 monary valve. 



The left auricle (Fig. 158) is not so large as the right, but its 

 walls are thicker. Discharging into it are the two right and the 



