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TEXT-BOOK OF PHYSIOLOGY 



termed a syncytium. As the heart develops the muscle-fibers make their 

 appearance in the protoplasm and assume an arrangement which corre- 

 sponds to that of the trabeculae composing the reticulum (Fig. 116). In 

 the adult heart the intermediary spaces are reduced to narrow clefts in 

 consequence of the multiplication of the muscle-fibers. The clefts are 

 occupied with connective tissue, blood-vessels, lymphatics, etc. The indi- 

 vidual fiber consists of alternate dim 

 lateral union an( j \{gfa bands similar to the corre- 



sponding bands of the ordinary skeletal 

 muscles, though it is devoid of a sar- 

 colemma. Among the fibers large oval 

 nuclei are distributed. At varying inter- 

 "vals the fibers are interrupted by inter- 

 calated discs. When the heart muscle 

 is treated with caustic potash the trabec- 

 ulae separate at the level of these discs, 

 forming what has hitherto been termed 

 the muscle cell or fiber. 



The arrangement of the muscle- 

 fibers is quite complicated and in ac- 

 cordance with the functions of the in- 

 dividual portions of the heart. In the 

 auricles the fibers are arranged in two 

 sets: an outer transverse set, which 

 pass from auricle to auricle, and an 

 inner longitudinal set, which pass over 

 the auricles and are attached anteriorly 

 and posteriorly to the connective tissue 

 of the transverse auriculo-ventricular 

 septum. The longitudinal fibers of the 



_ ._ LONGITUDINAL SEC- auricles are practically independent of 

 TION OF A, PAPILLARY MUSCLE OF THE HUMAN each other. Circularly arranged fibers 

 HEART, x 360. (Stokr.) are present near the terminations of 



the venae cavae and pulmonic veins. 



In the ventricles the muscle-fibers are also arranged in two sets, a 

 superficial longitudinal and a deep transverse, though their arrangement is 

 somewhat more complicated than that observed in the auricles. In a 

 general way it may be said that the superficial longitudinal fibers on both 

 the anterior and posterior surfaces take their origin in the connective tissue 

 of the auriculo-ventricular septum. The superficial fibers on the anterior 

 surface of the heart pass obliquely downward and forward from right to left 

 toward the apex, where they turn backward and inward in a vertical manner 

 after which they ascend to terminate in the wall of the septum, the columnae 

 carneae and musculi papillares. The superficial fibers of the posterior sur- 

 face of the heart pass obliquely downward from left to right, wind around 

 the apex, turn upward and end in the same structures as do the fibers from 

 the anterior surface. The fibers from the base of the right ventricle termi- 

 nate in the structures of the left ventricle, while those from the left ventricle 

 terminate in the structures of the right ventricle. Longitudinal fibers are 

 also found on the inner surface. The transverse fibers are very abundant 



Nucleus of Nucleus of Intercalated 



a muscle a connective disc. 



