THE CIRCULATION OF THE BLOOD. 



267 



lateral union 



striated muscle and the smooth muscle. A longitudinal section of the 

 heart-muscle shows a reticulated arrangement 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 

 appearance in the protoplasm and assume an arrangement which corre- 

 sponds to that of the trabeculae composing the reticulum (Fig. 121). 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 

 and light 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 disks. When the heart muscle 

 is treated with caustic potash the trabec- 

 ulae separate at the level of these disks, 

 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 

 auricles are practically independent of 

 ecah other. Circularly arranged fibers * R T F A 

 are present near the terminations of 

 the venae cavae and pulmonary 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 



Nucleus of Nucleus of Intercalated 

 a muscle a connective disc. 



fiber. tissue cell. 



FIG. 121. FROM A LONGITUDINAL SEC- 

 HUMAN 



