96 



MANUAL OF ELEMENTARY ZOOLOGY 



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membrane, the sarcolemma, which adheres at the end to 

 that of an adjacent fibre or to tendon. The cytoplasm 

 is characterised by alternate light and dark stripes which lie 

 across the length of the fibre (Fig. 59). The nuclei are scat- 

 tered throughout the substance of 

 the fibre, and often each is sur- 

 rounded by a little granular un- 

 differentiated protoplasm unlike 

 the modified protoplasm which 

 composes most of the fibre. 

 A longitudinal fibrillation is pre- 

 sent, but its relation to the stria- 

 tion is a matter of dispute. The 

 muscular tissue of the heart is a 

 peculiar kind of striped muscle 

 known as cardiac muscle. Its 

 cells, spindle-shaped in the frog, 

 short, square-ended cylinders in 

 Man, have each one nucleus, and 

 often a branch that abuts upon a 

 similar process of a neighbouring 

 cell. It is striped, but less dis- 

 tinctly so than ordinary muscle. 

 It is in this tissue that the auto- 

 matism of the heart resides. 

 Skeletal tissues are characterised 

 by a great develop- 

 ment of intercellular 

 or ground substance 

 between the cells, whereby it is 

 adapted to support and bind to- 

 gether other tissues. In cartilage 

 the ground substance is a homo- 

 geneous matrix readily stained 

 by silver nitrate. It is firm and 

 elastic, and sometimes toughened 

 by the development in it of fibres, 

 when it is known as fibrous 

 cartilage, in contradistinction to 

 plain or hyaline cartilage. The 

 cells are simple in shape and often 



Skeletal 

 Tissues. 



Fig. 55. — Medullated nerve 

 fibres, stained with osmic 

 acid and highly magnified. 



a.x.c. Axis cylinder ; m. s'. , med- 

 ullary sheath; ni^ neuri- 

 lemma; n.R., node of 

 Ranvier ; »«., nucleus. 



Note that the axis cylinder con- 

 tains longitudinal fibrils. 



