226 PHYSIOLOGY. 



tissue. The rings to which the valves are attached are also made 

 of endocardium and fibre-elastic tissue. 



Muscular Structure of the Heart. 



The muscular fibers of the auricles consist of two layers run- 

 ning in different directions. The external fibers are common to 

 both auricles, whilst some run into the interauricular septum. The 

 internal fibers are not common to both auricles, but are confined to 

 each auricle. The fibers of the internal layer are attached to their 

 respective auriculo-ventricular rings. The external fibers run in a 

 transverse direction; the internal fibers cross the direction of the 

 former. There are other muscular fibers, arranged concentrically 

 around the origin of the great veins and auricular appendages. 



In the ventricles there are several layers of muscles. The 

 outer layer runs from the base, where they are attached to the fibro- 

 cartilaginous rings around the orifices toward the apex of the heart, 

 where they run by a sharp 'twist into the interior of the left ven- 

 tricle to the papillary muscles. This twisting of the fibers gives 

 rise to the whorl of the fibers at the apex of the heart. Other 

 fibers run obliquely upward in the septum to be attached to the 

 fibro-cartilaginous ring, from which they started. Still other fibers 

 pass in a horizontal direction into the posterior wall of the left 

 ventricle and take a ring-like course in it. 



W. G. MacCallum found beneath the superficial fibers thicker 

 fibers which form three flat bands running in the form of a scroll 

 from one ventricle through the septum into the other. 



The right ventricle in the arrangement of its muscular fibers 

 may be regarded as an appendage of the left. 



Histology. The fibers of the heart are striated. Unlike the 

 voluntary muscle, they branch and have their ends united to each 

 other so as to form a network. The open space in the network is 

 filled with connective tissue and lymphatics. The muscle-cells are 

 quadrangular in shape, with clear oval nuclei. There is no sar- 

 colemma in heart-muscle. The muscles of the heart anastomose 

 and divide. As to lymphatics, the heart is very liberally supplied 

 with them. The nerves are nonmedullated near their ends. The 

 muscular mass of the heart is called the myocardium. 



Gibson 1 , in a histological study of the auricular muscles of the 

 heart, found three kinds of tissue: (1) those related to smooth mus- 



1 British Medical Journal, January 16, 1909. 



