THE VERTEBRATE BODY 



319 



the trigger. As explained above, it is the number of motor units that 

 contract that determines the strength of a muscle movement, rather than 

 a variable amount of contraction of the individual fibers. 



The second type of muscle in the frog's body is the smooth or vis- 

 ceral muscle. It is called smooth because, under the microscope, it ap- 

 pears as long slender cells, narrowing to a point at the end, with a single 

 nucleus, and without striations. The name visceral is given because it 

 is found primarily in the visceral organs. Smooth muscle is not as ef- 

 ficient as striated muscle ; smooth muscle cells can shorten themselves to 



Olfactory lobe 



Fore brain 



Mid brain 



Hind bran 



Cerebral hemispheres 



Diencephalon 

 Optic lobes 

 Cerebellum 

 - Fourth ventricle 

 Medulla 



Spinal cord 



Fig. 22.10. Dorsal view of the brain of a frog. 



one sixth of their original length when stimulated, while striated muscle 

 cells can contract to one tenth of their normal length. The smooth mus- 

 cles are mostly used to keep material moving through the alimentary 

 tract, through peristaltic contractions as first described in Chapter 13. 

 Their actions are controlled by branches of the autonomic nervous 

 system. 



The third type of muscle is cardiac muscle. This is found only in 

 the heart. By microscopic examination we learn that the fibers are long 

 and striated just as are those of skeletal muscles, but the fibers anasto- 



