320 



THE VERTEBRATE BODY 



mose freely ; that is, they form a continuous network in which cross-con- 

 nections join the fibers. The nuclei are in the center of the fibers rather 

 than along the edges as in skeletal muscles. The contractions of the 

 heart muscle are involuntary in nature as are those of smooth muscle, so 

 cardiac muscle shows a combination of the characteristics of the other 

 two. The heart muscle will continue to contract rhythmically even when 



Cerebrum 



Midbrain 

 z — Cerebellum 



Pons 



Medulla 



Spinal cord 



Fig. 22.11. Diagram of the parts of the brain of man. The parts have been separated 



somewhat for clarity. 



its nerves are severed. When the heart of a frog is held in the hand, it 

 will continue to pulsate for a considerable time. The ability to perform 

 rhythmical activities is a fundamental property of protoplasm which, in 

 cardiac muscle, has become greatly developed. The nerves that run to 

 this organ serve to accelerate or to inhibit the rate of heart beat in ac- 

 cordance with the needs of the animal's body, but they do not initiate the 

 beat. 



The muscles of man show no distinctive differences from those of the 

 frog, so there is no need for a separate discussion of them. Most of the 



