268 TEXT-BOOK OF PHYSIOLOGY 



From the foregoing facts physiologists frequently divide the general 

 circulation into: 

 r. The pulmonic circulation, which includes the course of the blood from 



the right side of the heart through the lungs to the left side of the 



heart. 



2. The systemic circulation, which includes the course of the blood from 



the left side of the heart through the aorta and its branches, through 

 the capillaries and veins, to the right side of the heart. 



3. The portal circulation, which includes the course of the blood from the 



capillaries of the stomach, intestines, pancreas, and spleen through 

 the portal vein, to the liver. 



Orifices and Valves. The movement of the blood along the path of 

 the circle above outlined is accomplished by the alternate contraction and 

 relaxation of the muscle walls of the heart. That the movement may be a 

 progressive one, that there shall be no regurgitation during either the con- 

 traction or the relaxation, it is essential that some of the orifices of the 

 heart be closed during each of these periods. This is accomplished by the 

 heart valves. 



The right auriculo-ventricular opening is surrounded and strengthened 

 by a ring of fibrous tissue to which is attached a membrane partially sub- 

 divided into three portions or cusps, for which reason it is] known as the 

 tricuspid valve; during the period of relaxation these cusps are directed 

 into the ventricle (Fig. no); during the period of contraction they are 

 raised and placed in complete apposition, when they act as a valve pre- 

 venting a backward flow into the auricle (Fig. in). In the former posi- 

 tion the orifice is open; in the latter it is closed. This valve' is formed of 

 fibrous tissue derived from the fibrous ring, and some muscle-fibers, and 

 covered over by a reduplication of the endocardium. To the under surface 

 and to the edges of this valve the tendinous cords of the papillary muscles 

 are firmly and intricately attached. These cords are just sufficiently long 

 to permit apposition of the cusps and to prevent them from being floated 

 into the auricle. 



The left auriculo-ventricular opening is provided with a similar though 

 better developed fibrous ring and membranous valve. It is, however, 

 subdivided into but two portions or cusps, for which reason it is known as 

 the bicuspid valve, or, from its fancied resemblance to a bishop's mitre, the 

 mitral valve. The general arrangement, connections, and mode of action 

 of this valve are similar in all respects to those of the tricuspid valve. 



The orifice of the pulmonic artery is also surrounded by a ring of 

 fibrous tissue to which are attached three semilunar or pocket-shaped mem- 

 branes, the semilunar valves. Each valve is formed by a reduplication of 

 the endocardium strengthened by fibrous tissue. In the center of the free 

 edge of the valve there is a small nodule of fibro-cartilage (the corpus 

 Aurantii). The outer edge of the valve is strengthened by a delicate fib- 

 rous band. A similar band strengthens the convex attached portion of^the 

 valve just where it is joined to the fibrous ring. A third set of fibers pass 

 toward the nodule, interlacing in all directions. Two narrow crescentic- 

 shaped areas (the lunulae) near the free edge are devoid of these fibers. 

 During the period of relaxation of the heart the edges^of the valves are in 

 close apposition and prevent a return of the blood into' the ventricle (Fig. 



