46 



PHYSIOLOGY OP MUSCLE AND NEEVE 



Cardiac muscle tissue occupies a special position, because embryologically as 

 well as histologically it appears in the form of modified contractile fibers. This 

 is especially evident in the lower vertebrates in which these cells possess a spindle- 

 shape, a marked cross-striation, and a long-oval nucleus. In mammals, the cardiac 

 muscle cell appears as a short cylinder which is usually united with a neighbor- 

 ing one by an oblique process to form a muscular plexus. 1 Functionally it is of 

 interest to remember that these prolongations bring the cells of adjoining rows or 



B 



FIG. 17. FIG. 18. 



FIG. 17. FIBRILS OF THE WING MUSCLES OF A WASP, PREPARED BY ROLLETT'S 

 METHOD. HIGHLY MAGNIFIED. 



A, a contracted fibril; B, a stretched fibril with its sarcous elements separated at the 

 line of Hensen; C, an uncontracted fibril showing the porous structure of the sarcous 

 elements. (Schafer.) 



FIG. 18. SMOOTH MUSCLE CELLS, TEASED APART AND SHOWING LONG OVAL 

 NUCLEI SURROUNDED BY UNDIFFERENTIATED PROTOPLASM. 



areas into closer relation. The oval nucleus occupies a position in the axial portion 

 of the cell which also contains much undifferentiated protoplasm, or sarcoplasm. 

 The other parts of the cell exhibit a very delicate cross-striation. 



The Action of Striated Muscle in Locomotion. As far as the 

 mechanical properties of the resting muscle are concerned, we have 

 previously seen that it is a very yielding tissue and possesses a soft 

 consistency so that its shape may be varied with ease. The contracted 

 muscle, on the other hand, is firm to the touch and exhibits a more 

 rounded outline, because its length is diminished in favor of its breadth. 



1 Zimmermann, Uber den Bau der Herzmuskulatur, Archiv fur mikr. Anat., 

 Ixxv, 1910. 



