2o Introduction to Animal Morphology. 



C H AFTER IV. 



ANIMAL TISSUES. 



MUSCULAR tissue consists of contractile cells. While 

 in the simplest cells the unaltered protoplasm alone 

 is contractile, here the entire cell is so. The simplest 

 muscle cell is fusiform, simple or forked* at its ends, 

 ao" to ^V'long, with its thickest part not quite cen- 

 tral, and containing a rodlike nucleus capped by a 

 pyramidal mass of granules at its ends (Klcbs). When 

 it contracts, the cell surface shows transverse,f and oc- 

 casionally longitudinal, striae. Such cells joined by 

 connective tissue form fascicles of involuntary muscle, 

 such as are found in the intestinal wall of Vertebrates, 

 or in the body wall of all classes, except Arthropoda, 

 from Sponges upwards. Voluntary or striped fibres 

 consist of conjoined contractile cells within a 

 connective [some say elastic] sheath (Sarcolemma), 

 the contractile matter being in discs separated by 

 clear interspaces. Each fibre may be split artificially 

 into obliquely inclined transverse discs (cleavage 

 planes j or longitudinal fibrils (consisting of single 

 rows cf square sarcous elements, large and small 

 alternately, with clear interspaces). || The sizes of 



* As in the fibres of the Frog's bladder. Striped muscles ending in the 

 skin divide into pointed fibrils which appear continuous with the processes 

 of the connective tissue corpuscles of the dermis (Salter). 



t Meissner. 



Wagener. 



The dark bands (disdiaclasts) consist of granules of a double refracting 

 substance in the general isotropic myosin fluid substance of the fibre. 



|| Intermediate forms exist between the smooth and striped fibres : thus 

 in the involuntary fibres of the bladder Schwalbe saw a partial striation, and 



