THE CIRCULATION OF THE BLOOD 



269 



no); during the contraction they are directed into the artery (Fig. in). 

 In the former position orifice is closed; in the latter, it is open. 



The orifice of the aorta is also surrounded by a ring of fibrous tissue 

 to which are attached three semilunar or pocket-shaped valves, which 

 in their arrangement, connection, and mode of action are similar in all 

 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 the valves when closed are represented in Fig. 112. 



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 the 

 heart-muscle shows a reticulated ar- 

 rangement of the fibers, the outcome 

 of a similar reticulated condition of 

 the mesodermic material in which they 

 develop. The mesodermic reticulum 

 containing numerous nuclei is termed 

 a syncytium. As the heart develops 

 the muscle-fibers make their appear- 

 ance in the protoplasm and assume an 



fcapil- 

 lary 



lateral union 



FIG. ii2. VALVES OF 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. ( 

 amy and Beau.) 



Nucleus of Nucleus of Intercalated 

 a muscle a connective disc, 



fiber. tissue cell. 



FIG. 113. FROM A LONGITUDINAL SEC- 

 TION OF A PAPILLARY MUSCLE OF THE HUMAN 



HEART.?' *X 360. (Stohr.) 



arrangementjwhich corresponds to that of the trabeculae composing the re- 

 ticulum (Fig. 113). In the adult heart the intermediary spaces are re- 

 duced to narrow clefts in consequence of the multiplication of the muscle- 

 fibers. The clefts are occupied with connective tissue, blood-vessels, lym- 

 phatics, etc. The individual fiber consists of alternate dim and light bands 

 similar to the corresponding bands of the ordinary skeletal muscles, though 

 it is 'devoid of a sarcolemma. Among the fibers large oval nuclei are dis- 

 tributed. At varying intervals the fibers are interrupted by intercalated 

 discs. When the heart muscle is treated with caustic potash the trabeculae 

 separate at the level of these discs, forming what has hitherto been termed 

 the muscle cell or fiber. 



