

es) .-"^ 



/ Voluntary 

 / muscle 

 cells 



Involuntary- 

 muscle cells 



VOLUNTARY AND INVOLUNTARY MUSCLES 



Muscles attached to the bones and skin consist of cells that appear to be striped 

 when seen with a microscope. They are connected with the brain or the spinal cord 

 and are subject to voluntary control. Muscles of the blood vessels and the viscera 

 are not striped; they are all involuntary muscles 



discharged into the blood. Now the chemical condition of the blood di- 

 rectly stimulates the vagus nerve, which in turn acts upon the heart and 

 the breathing. The adjustment of the pulse rate and breathing rate to chang- 

 ing conditions of the blood is thus almost immediate. 



Because of its many connections with all the organs of the body, the 

 autonomic nervous system ties all the parts together so that they act as a 

 whole through the reflexes. The autonomic system includes in its control, 

 however, much more than involuntary muscles. Some of the endocrine 

 secretions (see page 313) act upon the autonomic nervous system; this in 

 turn acts upon some of the endocrine glands. 



296 



