SYSTEMATIC SYNTHESIS OF TISSUES INTO ORGANS. 283 



the contracting muscular fibre, and then exerts an influ- 

 ence through the nerve, is not known. 



701. Remark. IT is NOT POSITIVELY CERTAIN that there are two 

 kinds of nerves connecting between a muscle and the centres, but it is 

 supposed there are, because it does not appear probable that the motor 

 influence can be acting toward the muscle, and the sensatory from it 

 through the same nerve-fibre at the same time ; and if the back roots of 

 the spinal nerves be cut, reflex motion cannot be excited by acting on 

 the muscle. This seems conclusive, although all the nerve-fibres in the 

 muscle appear alike ; motory and sensatory cannot be distinguished 

 there. 



702. All that is KNOWN WITH CERTAINTY is that 

 every contraction and relaxation of any part of even the 

 smallest fibrilla of the voluntary muscles can instantly 

 produce a sensation, and also other exhilarating or de- 

 pressing effects upon, or through the effect of, many nerv- 

 ous centres, waking the whole body to life and joy, or 

 overwhelming it with gloom and misery. 



703. It IS OUR DUTY TO STUDY AND OBSERVE the ef- 



fects of over and under exercising the muscles, the fatigue, 

 exhaustion, and dragging sensations produced by over 

 toil, and the discomfort, unrest, uneasiness, and impul- 

 siveness, that is caused by a want of muscular exercise. 

 A regularly, healthily exercised boy is very little inclined 

 to mischief. 



T/ie SJcin as an Organ of Sense. 



704. Forming the surface of the body, IT is NECES- 

 SARY THAT THE SKIN BE ENDOWED with the power of 



exciting sensations. It must be an advance picket-guard, 

 to report upon the presence of objects in contact with 

 the body, and upon the temperatures to which it is ex- 

 posed, and must excite a sudden alarm if injured in any 

 way. 



705. THE SKIN is CONSTRUCTED of three layers, 



701. What is ? 702. What is ? 703. What is ? What said of regain* 

 exercise ? 704. With what ? 705. How is ? 



