CHAPTER XVIII. 



NERVOUS TISSUE. THE CEREBRO-SPINAL SYSTEM: BRAIN AND 

 CRANIAL NERVES; SPINAL CORD AND SPINAL NERVES. 

 SYMPATHETIC SYSTEM. 



Nervous tissue. Nervous tissue is the most highly organized 

 tissue of the body. It is pre-eminently the "master tissue," 

 controlling, regulating, and directing all the other tissues. 

 Like muscular tissue, it is irritable and responds to stimuli; 

 but, unlike muscular tissue, it is not contractile. 



If we stimulate a nerve-fibre, certain molecular changes are 

 started in the fibre, and these changes are transmitted along 

 the fibre without visibly altering its form. We call these 

 changes thus propagated along a fibre, "nervous impulses." 

 The stimuli that start these nervous impulses are varied and 

 numerous, and may originate from within the body as well as 

 from without. They usually originate from without; as, for 

 example, a ray of light falls on the nervous tissue of the eye, 

 the light stimulates and sets up changes in the nervous tis- 

 sue which are transmitted along the optic nerve to the brain. 

 These nervous impulses reaching the brain may start other 

 nervous impulses, which, travelling down certain nerves to 

 certain muscles, will cause those muscles to contract, in which 

 case we say a man starts. Again, sound falling on the ear, a 

 drop of water falling on the hand, some change in the air or 

 other surroundings of the body, or some change within the body 

 itself, may so affect the nervous tissue that nervous impulses 

 are started and travel to this point or that, giving rise to move- 

 ment as they reach muscular tissue, or producing some other 

 effect as they reach other tissues. We may say that nervous 

 tissue generates, transforms, and propagates nervous impulses. 



When subjected to the microscope, nervous tissue is seen to 



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