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HANDBOOK OF PHYSIOLOGY. 



The muscular fibres of the heart, unlike those of most of the invol- 

 untary muscles, are striated ; but although, in this respect, they TC- 

 semble the skeletal muscles, they have distinguishing characteristics of 



their own. The fibres which lie side by 

 side are united at frequent intervals 

 by short branches (Fig. 92). The fibres 

 are smaller than those of the ordinary 

 striated muscles, and their striation is 

 less marked. No sarcolemma can be 

 discerned. The muscle-corpuscles are 

 situate in the middle of the substance 

 of the fibre ; and in correspondence 

 with these the fibres appear under cer- 

 tain conditions subdivided into oblong 

 portions or "cells," the offsets from 

 which are the means by which the fibres anastomose one with another 

 (Fig. 93). 



Endocardium. As the heart is clothed on the outside by a thin 

 transparent layer of pericardium, so its cavities are lined by a smooth 

 and shining membrane, or endocardium, which is directly continuous 



FIG. 91. Transverse section of bul- 

 lock's heart in a state of cadaveric rigid- 

 ity, a, cavity of left ventricle, b, cavity of 

 right ventricle. (Dalton.) 



Fio. 92. 



FIG. 93. 



Fio. 92. Network of muscular fibres (striated) from the heart of a pig. The nuclei of the 

 muscle-corpuscles are well shown, x 450. (Klein and Noble Smith.) 

 FIG. 93. -Muscular fibre cells from the heart. (E.A. Schafer.) 



with the internal lining of the arteries and veins. The endocardium is 

 composed of connective tissue with a large admixture of elastic fibres ; 

 and on its inner surface is laid down a single tesselated layer of flattened 



