TISSUES AND ORGANS 61 



solution that are formed by the destruction of 

 muscle substance, or of chemical matters 

 in it, during contractile activity. Contractile 

 tissue constantly requires oxygen, and it 

 constantly produces carbonic acid ; this is 

 so, even during the resting of muscle, but, 

 when it contracts, much more oxygen is used 

 and much more carbonic acid is produced. 

 So complicated are the chemical processes 

 in contractile tissue that there is still uncer- 

 tainty regarding them. 



28. The controlling and regulating tissue is 

 nervous tissue. It is found in the brain, 

 spinal marrow, ganglia, parts of the organs 

 of sense, and in nerves, and it consists of 

 nerve fibres and nerve cells. These will be 

 more fully considered when we treat of 

 nervous actions. Suffice it to say here 

 that nerve fibres generate and conduct what 

 we term a nervous impulse, a change, the 

 true nature of which we do not know. 

 Such an impulse, issuing from a nerve 

 centre, such as a portion of the spinal 

 cord, may travel to a muscle and cause the 

 striated tissue to shorten, when there may 



