NORMAL HISTOLOGY. 



When examined in sections passing parallel to the general course of 

 the fibres (Fig. 69), the heart-muscle exhibits a close irregular reticulum of 

 anastomosing trabeculae. The latter consist of strands of striated muscle in 



which lie oval nuclei, surrounded by pale areas 

 of undifferentiated granular cytoplasm, or sarco- 

 P/asm, devoid of striations. In cross-sections 

 (Fig. 70), the contractile fibrillae are seen to be 

 arranged in radiating groups which occupy the 

 periphery of the trabeculae but do not reach 

 inward as far as the axis. The latter consists of 

 a variable core of sarcoplasm, which surrounds 

 the nucleus and usually contains a small quantity 

 of pigment and fatty particles. In its general 

 histological details, cardiac muscle agrees with 

 typical striped muscle, the alternate light and 

 dark stripes depending upon similar variations 



r\~tia* 



Undifferentiated,. *"" jTVV 



blood- 

 vessel 



sarcoplasm 



FIG. 69. Muscle-fibres of human 

 heart. X 375- 



FIG. 70. Fibres of cardiac muscle in transverse section. 

 X375- 



of density along the component fibrillae. The probable significance of these 

 markings will be considered under Striated Muscle (page 58) ; suffice it here to 

 indicate the peculiarities in which the muscular tissue of the heart differs from 

 typical striated muscle. Although invested by a delicate sheath, the cardiac 

 fibres do not possess a well defined sarcolemma^- Their longitudinal striation,' 

 often very distinct, owing to the large amount of sarcoplasm between the 

 groups of fibrillae, is seemingly interrupted at intervals by dark transverse 

 markings, the intercalated disks. Examination of suitably prepared material, 

 under high power, shows, however, that the contractile fibrillae do not surfer 

 interruption, but are continued without break across the intercalated disks. 

 Further, the disks do not lie on the same plane, but at different levels. After 

 dissociation reagents, such as a solution of caustic potash, heart-muscle breaks 

 up into irregularly branched pieces, the so-called fibres. The lines of fracture 

 correspond in position with the intercalated disks and, consequently, the ends 

 of the isolated fibres often are not straight, but exhibit a series of offsets like 

 steps. The incessant contraction of the cardiac muscle is reflected in its 

 structure, particularly in the fibres of Purkinje (page 102), the unusually 

 large amount of sarcoplasm recalling a similar condition observed in the 

 " red " muscles (page 59), in which a lower degree of differentiation seems 

 associated with the power of enduring frequently repeated contraction. 



