6 NORMAL HISTOLOGY. 



pointed or rounded extremities, while the sarcolemma is united with 

 the endomysium of the surrounding fibres. The muscle-substance 

 is never directly continuous with adjacent tissues, but is always 

 enclosed within the sac of the sarcolemma ; the union between the 

 fibres and other structures is effected by the blending of the endo- 

 mysium of the muscle-fibres with the connective tissue of the attach- 

 ments, whether these be tendon, periosteum, perichondrium, or 

 subcutaneous tissue ; the sarcolemma closely invests the sarcous 

 contents, being simply received into the connective tissue without 

 becoming directly continuous. 



CARDIAC MUSCLE. 



The muscular tissue of the heart, as well as of the cardiac ends of 

 the large veins, forms an intermediate group of contractile tissue, 

 standing in its development between the simple 

 FlG - 7 8 - spindle non-striated cell on the one hand and 



the highly differentiated striped fibre on the 

 other. Among the lower vertebrates (fishes, 

 amphibians) the cardiac muscle is composed 

 of nucleated spindle-cells possessing distinct 

 ; 'i ' . ! J ! transverse striations and often branched ends ; 

 in man and the higher vertebrates these spindle- 

 cells give place to short, striated, cylindrical 

 fibres, provided with lateral processes. By the 

 apposition of these richly-branched cells a 

 close, narrow-meshed net-work is formed, the 

 Heart - muscle, Bowing j uncture between the individual elements being 



several joined branched ... -. - 



fibres: around the poles of indicated by transverse lines of cement-sub- 



the nuclei are aggregations stance 

 of pigment-granules. 



The peculiarities of heart-muscle are 



1. The absence of the sarcolemma, the transversely striated 

 and more faintly longitudinally marked muscular tissue being 

 naked. 



2. The situation of muscle-nuclei within the sarcous substance, 

 usually nearThe centre of the cell. 



3. The characteristic arrangement of the contractile fibrillae, since 

 these are so placed that the peripheral fibrillae are grouped into flat, 

 ribbon-like muscle-columns, somewhat radially disposed about the 

 circumference of the fibre ; the remaining central portion is occupied 

 by prismatic bundles of fibrillae, together with the nuclei and the 

 associated protoplasm (Ranvier, Kolliker). The small masses of pro- 

 toplasm which surround the muscle-nuclei usually contain minute 

 fat-drops and pigment-granules. The amount of pigment normally 

 present varies with age, increasing from the tenth year (Maass). 



