THE ANIMAL MACHINE 



The strictly involuntary muscles, however, are placed 

 absolutely beyond control of the will. The most im- 

 portant of these muscles are those that constitute the 

 heart and the diaphragm, and that enter into the 

 substance of the walls of blood vessels, and of the 

 abdominal organs. It is obvious that the functioning 

 of these important organs could not advantageously be 

 left to the direction of the will; and so, in the long 

 course of evolution they have learned, as it were, to 

 take care of themselves, and in so doing to take care 

 of the organism, to the life of which they are so abso- 

 lutely essential. As the physiologist views the matter, 

 no organism could have developed which did not 

 correspondingly develop such involuntary action of 

 the vital organs. It will be seen that the involuntary 

 muscles differ from the voluntary muscles in that they 

 are not connected with bones. Instead of being 

 thus attached to solid levers, they are annular in struc- 

 ture, and in contracting virtually change the size of the 

 ring which their substance constitutes. Each fibre 

 in contracting may be thought of as pulling against 

 other fibres, instead of against a bony surface, and the 

 joint action changes the size of the organ, as is obvious 

 in the pulsing of the heart. 



Though the rhythmical contractions of the involuntary 

 muscles are independent of voluntary control, it must 

 not be supposed that they are independent of the con- 

 trol of the central nervous mechanism. On the con- 

 trary, the nerve supply sent out from the brain to the 

 heart and to the abdominal organs is as plentiful and as 

 important as that sent to the voluntary muscles. There 



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