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HAND-BOOK OF PHYSIOLOGY. 



tary muscles, are striated; but although, in this respect, they resemble 

 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. 95). 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 appeal- 

 under certain conditions subdivided into oblong portions or "cells," the 

 off-sets from which are the means by which the fibres anastomose one 

 with another (Fig. 96). 



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 



FIG. 95. 



FIG. 



FIG. 95. Network of muscular fibres (striated) from the heart of a pig. The nuclei of the mus- 

 cle-corpuscles are well shown. X 450. (Klein and Noble Smith.) 

 FIG. 96. Muscular fibre cells from the heart. (E. A. Shafer.) 



shining membrane, or endocardium, which is directly continuous 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 tessellated layer of flattened endothelial cells. 

 Here and there unstriped muscular fibres are sometimes found in the tis- 

 sue of the endocardium. 



Course of the Blood through the Heart. The arrangement of 

 the heart's valves is such that the blood can pass only in one direction, 

 and this is as follows (Fig. 97): From the right auricle the blood passes 

 into the right ventricle, and thence into the pulmonary artery, by which 

 it is conveyed to the capillaries of the lungs. From the lungs the blood, 

 which is now purified and altered in color, is gathered by the pulmonary 



