Dynamic Theory. 

 other charge from the auricle; and the blood just driven up the pul- 

 monary artery is prevented from coming back into the ventricle by three 

 fleshy flaps hung to the sides like swinging gates, which are instantly 

 closed by the weight of the blood falling back toward the ventricle. 

 These are called semilunar valves, from their half-moon shape. After 



FIG. 242. Interior of the right auricle and 

 ventricle, exposed by removal of a part of 

 their right and anterior walls. 



l. Superior vena cava. 



2. Inferior vena cava. 



2 .Hepatic veins. 



3. Septum separating the auricles. 



3 .Oval fossa. (In the foetus there is a 

 hole at this place between the two auri- 

 cles.) Just below it is the Eustachian valve. 



3 .Entrance of the great coronary vein, 

 with its valve. 



+ +. Auriculo Ventricular groove, 

 with a narrow strip of the adjacent walls 

 of the auricle and ventricle remaining. 



4, 4. On the septum in the cavity of right 

 ventricle. 



4'. Large anterior papillary muscle to 

 which chordae tendinse are attached. 



5, 5 , 5 ".Left, right and posterior or sep- 

 tal segments of tricuspid valve. 



6. Inside the pulmonary artery part of 

 the front wall having been removed. Be- 

 low the figure is a strip of the wall and the 

 pulmonary valve. 



7. On underside of aortic arch close to 

 the cord of the ductus arteriosus. 



8. Ascending part of the aortic arch 

 the lower part is covered by the appendix 

 of the auricle and the pulmonary artery. 



9. Placed between the innominate and 

 left common carotid arteries. 



10. Appendix of the left auricle. 



11, 11. Left ventricle. 



(Allen Thomson.) 



passing through the lungs the blood returns down the pulmonary vein 

 and enters the left auricle. This vessel then contracts and drives the 

 blood through the mitral valve into the left ventricle. The mitral 

 valve consists of two triangular flaps, one on each side of the passage ; 

 and when they fall together they close the way against the return of the 

 blood into the left auricle upon the contraction of the ventricle, which it 

 tends to do. The mitral valve is provided with the supporting cords, 

 like those of the tricuspid. When the left ventricle contracts, the blood 

 is forced into the aorta, and through the arteries into the general sys- 

 tem. The ventricle dilates and is filled again from the left auricle, 

 while the blood in the aorta is barred from returning to the ventricle by 

 semi-lunar valves, like those in the pulmonary artery. 



The heart is composed of muscle fibres, striated like those of the ac- 

 tive muscles of the limbs, &c. But the fibres are destitute of the sar- 

 colemma, or sheath, which striated muscles usually have. These fibres 

 are in layers in the different parts of the heart, some lying in one direc- 

 tion and some in another. The walls of the different parts vary from 

 each other in thickness. The average thickness in adult men of the 

 right auricle is one-twelfth of an inch, or one line. The right ventricle 

 averages about two lines, the left auricle about one line, and the left ven- 



