218 MENTAL AND PHYSICAL EXERCISE. 



voluntary powers, no necessary use appears to be made 

 of this communication ; since \ve can employ the mus- 

 cles of one side, without the necessary action of those of 

 the other, or we can use them both at the same time. 

 Thus we can throw up one hand, or both, at the same in- 

 stant. 



Insensibility of the Brain. The brain, as we have 

 seen, is the source of every sensation ; the common sen- 

 sorium through which we derive every pleasure, and feel 

 every pain. And yet this wonderful organ, so sensible 

 to mechanical impressions that a little blow, even through 

 the bones of the skull, will often produce instant death, is 

 itself entirely insensible ! "That part of the brain," says 

 Sir Charles Bell, " which if disturbed or diseased, 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 exquis- 

 itely sensible. 



OF THE MUSCLES. 



\ 



We have designedly omitted to treat of the muscles, 

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

 with propriety be described in connection with an ac- 

 count 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. 



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 volunta- 

 ry muscles, in order to distinguish them from the heart, 

 and other muscular parts over which the will has no con- 

 trol. Every muscle of course, is furnished with its ap- 

 propriate set of nerves. 



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

 muscles consist of 1 Why are they called voluntary muscles ? 



