102 



METAZOA IN GENERAL 



Cell 

 body 



D end rites 



virtue of their power to contract; their subsequent elongation is simply 

 a matter of relaxation and returning to a normal shape. 



Three types of muscle tissue are recognized: (1) skeletal, striated and 

 voluntary, found, generally speaking, 

 in the muscles which are themselves 

 organs under the control of the will 

 and often attached to the skeleton; (2) 

 visceral, nonstriated and involuntary, 

 generally not under the control of the 

 will and forming a part of other organs; 

 and (3) heart WMscZe, a type intermediate 

 between the two others, found in the 

 heart (Fig. 41). Skeletal muscles con- 

 sist of large multinucleated fibers which 

 show a very marked cross banding or, 

 as it is termed, cross striation; visceral 

 muscle fibers are individual cells and 

 do not show this cross striation; heart 

 muscle is made up of individual cells 

 which are involuntary but cross-striated. 



Nucleus 



Confracfile fibril 

 A 



Sarcoplasm 



i 



Cytoplasm 

 Nucleus 



B 



Fig. 41. 



Collateral 



Neurilemma 



Nucleus of 

 neurilemma 



Motor 



end 



plorf-e 



^y^i Muscle 

 fiber 



Fig. 42. 



Fig. 41. — Different types of muscle cells. A, portion of a striated muscle fiber showing 

 a section in which the contractile fibrils are divided into groups by semifluid sarcoplasm. 

 Two nuclei are shown, surrounded by undifferentiated cytoplasm, and the whole fiber is 

 surrounded with a delicate sheath, or sarcolemma. B, three nonstriated muscle fibers, or 

 cells. C, several cardiac muscle cells. All highly magnified. 



Fig. 42. — Diagram of a nerve cell, possessing a cell body and a medullated motor nerve 

 fiber, ending in a motor end plate. Such cells are characteristic of the spinal cord of 

 vertebrates. The medullary sheath is acquired while the axon is in the outer layers of the 

 cord, and the neurilemma as the fiber emerges from the cord. The nerve fiber is too long 

 to be shown entirely, so a break is indicated. 



Of these types of muscle tissue, nonstriated muscles are found more 

 generally in the lower animals and the striated muscles predominate in 

 the higher forms. In the higher forms the nonstriated muscles are in 



