OF THE NERVOUS SYSTEM. 409 



nerve fibre, given off as a branch from the terminal Fig. 141. 



plexus, approaches the muscular fibre, usually at 

 a right angle, and penetrates its exterior; the 

 tubular sheath of the nerve fibre becoming con- 

 tinuous with the sarcolemma. At the same time 

 its medullary layer ceases abruptly, and the axis 

 cylinder spreads out into a thin oval expansion 

 of granular matter interspersed with nuclei, called 

 the " terminal plate," and lying in immediate con- 

 tact with the contractile substance of the muscular TERMINATION OF A 



fibre. Some variations in the form and disposi- NERVE FIBRE in mus- 

 cular fibre, from the fowl, 

 tion of the axis cylinder in the terminal plate are (R OU get.) 



to be seen in the muscles of amphibia ; but the 



above represents its essential characters in the muscles of birds and 



mammalians. 



Physiological Properties of the Nerve Fibres. The nerve fibres are 

 organs of communication. They serve as connecting filaments between 

 the nervous centres on the one hand and the peripheral organs of sensa- 

 tion and motion on the other. For this purpose they are endowed with 

 a power of irritability by which, when excited at one or the other 

 extremity, they transmit the nervous impulse throughout their entire 

 length, and produce a corresponding effect at their opposite termination. 

 Thus the nerve fibres distributed to the skin, when excited at their 

 peripheral extremities, produce in the brain a sensation corresponding 

 to the external impression. On the other hand, those which are distri- 

 buted to the muscles, when excited at their origin by the impulse of the 

 will, produce a contraction in the muscular fibres at their periphery. 

 This physiological action produces no visible change in the nerve fibre 

 itself. Its effects are manifest only at the extremities of the nerve, in 

 the organs where it terminates. Nevertheless, it is evident that the 

 nerve fibre serves to communicate in some way an action from one of 

 its extremities to the other ; since, if it be divided in any part of its 

 course, the communication at once ceases, and sensation can no longer 

 be perceived from impressions made upon the skin, nor voluntary con- 

 traction excited in the muscles. 



Owing to the different effects thus produced, at their central and peri- 

 pheral extremities, the nerve fibres and the nerves composed of them 

 have been distinguished by different names. Those which transmit the 

 stimulus of sensation, from the periphery to the nervous centres, are 

 called sensitive nerves or nerve fibres ; those which transmit the stimulus 

 of motion, from the nervous centre outward to the muscles, are called 

 motor nerves or nerve fibres. As a general rule, both sensitive and 

 motor nerve fibres are associated together in the same nervous bundle, 

 and separate from each other only when near their final distribution in 

 the skin or mucous membranes on the one hand, and in the muscles on 

 the other. But in some situations, near the origin of the nerves as well 

 as near their termination, the sensitive and motor fibres run in distinct 

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