214 MENTAL AND PHYSICAL EXERCISE. 



of the voluntary powers, no necessary use appears to be 

 made of this communication ; since we can employ the 

 muscles of one side without the necessary action of those 

 of the other, or we can use both of them at the same 

 time. Thus we can throw up one hand or both, at the 

 same instant. 



665. Insensibility of the Brain. The brain, as we 

 have seen, is the source of every sensation ; the common 

 sensorium through which we derive every pleasure, and 

 feel every pain. And yet this wonderful organ, so sensi- 

 ble to mechanical impressions, that a little blow, even 

 through the bones of the scull, will often produce instant 

 death, is itself entirely insensible ! " That part of the 

 brain," says Sir Charles Bell, " which if disturbed or dis- 

 eased, takes away consciousness, is as insensible as the 

 leather of our shoe !" It may be touched, or a portion 

 of it torn off, without sensation, and yet to its proper office 

 it Is exquisitely sensible. 



THE MUSCLES. 



666. We have designedly omitted to treat of the mus- 

 cles, until we came to that part of our work where they 

 could with propriety be described in connection with an 

 account of their functions, the exercise of which is one of 

 the principal means by which we are to continue in the 

 enjoyment of health, both corporeal and mental. 



667. The muscles are the red fibrous parts of animals, 

 which are situated immediately under the skin. They 

 constitute all those parts commonly called flesh. Their 

 number in the human body is about 400. They consist 

 of distinct portions* or separate bundles of fibres, which 

 are susceptible of contraction and relaxation, at the will 

 of the animal ; for which reason they are called voluntary 

 muscles, in order to distinguish them from the heart and 

 other muscular parts, over which the will has no control. 

 Every muscle, of course, is furnished with its appropriate 

 set of nerves. 



What are the muscles ? What do they constitute ? What do the mus- 

 cles consist of? Why are they called voluntary muscles ? 



