THE CIRCULATION OF THE BLOOD 



269 



preventing a backward flow into the auricle (Fig. 114). In the former 

 position the valve is open; in the latter, shut. For these reasons this struc- 

 ture is known as the tricuspid valve. 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 closure 

 of the valve and to prevent its being floated into the auricle. 



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 

 fibrous 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 nod- 

 ule, 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 ap- 

 position and prevent a return of 

 the blood into the ventricle (Fig. 

 113); during the contraction they 

 are directed into the artery (Fig. 

 114). In the former position they 

 are shut; in the latter, they are open- 



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

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

 subdivided into but two portions or cusps, and is therefore termed 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 aorta is also surrounded by a ring of fibrous tissue to which 

 are attached three semilunar or pocket-shaped valves (Fig 112) which in 

 their arrangement, connections, and mode of action are similar in a 1 respects 

 to those at the orifice of the pulmonic artery. The anatomic relations of 

 the cardiac orifices one to the other and the appearance presented by t 

 valves when closed are represented in Fig. 115- 



The Heart Muscle-fibers and Their Arrangement.- The muscle- 

 fibers of the heart represent in their structure a type between the ordinary 

 striated muscle and the smooth muscle. A longitudinal section of 

 heart-muscle shows a reticulated arrangement of the fibers the outcome of 

 a similar reticulated condition of the mesodermic material in which hey 

 develop The mesodermic reticulum containing numerous nuclei 



FIG. 1 1 5. VALVES or THE HEART, i. Right 

 auriculo-ventricular orifice, closed by the tri- 

 cuspid valve. 2. Fibrous ring. 3. Left auric- 

 ulo-ventricular orifice, closed by the mitral 

 valve. 4. Fibrous ring. 5. Aortic orifice and 

 valves. 6. Pulmonic orifice and valves. 7,8,9. 

 Muscular fibers. (Bonamy and Beau.) 



