352 



THE NEKVOUS SYSTEM 



FIG. 90: 



fibre, its effects being manifest only in the organs where it terminates. 

 Nevertheless, it is evident that the fibre serves to communicate in 

 some way an action from one extremity to the other ; since, if it be 

 divided in any part of its course, the communication ceases, and sensa- 

 tion can no longer be perceived from im- 

 pressions made on the skin, nor voluntary 

 contraction excited in the muscles. 



Owing to the different effects thus pro- 

 duced at their extremities, the nerves and 

 nerve fibres are distinguished by different 

 names. Those which transmit the stimu- 

 lus of sensation, from the periphery to 

 the centre, are called sensitive nerves or 1 

 nerve fibres ; those which transmit the i 

 stimulus of motion, from the nervous 

 centre to the muscles, are called motor 

 nerves or nerve fibres. As a rule, both 

 sensitive and motor fibres are associated 

 in the same bundle, and separate from 

 each other only near their final distribu- 

 tion. But in some situations, near the 

 origin of the nerves as well as near their 

 termination, the sensitive and motor fibres 

 run in distinct bundles ; as, for example, 



NERVOUS TERMINATION IN A Muscu- . ,. ,, /./..i /> i 



LAR FIBRE OF THE GREEN LIZARD, m the two roots of the fifth pair oi cranial 

 A, sheath of Henie, surrounding nerves, and in those of the spinal nerves 



the nerve fibre. 6, Annular constric- m , 1 , , , 



tion, and division of the nerve fibre, generally. The fibres belonging to the 

 m. Last interannuiar segment r, facial nerve are all motor fibres, making 



Terminal arborization of axis cylin- , , . i i mi, 



der beneath the sarcolemnia. (Ran- thlS exclusively a motor nerve. Those 



vier -) branches of the fifth pair, on the other 



hand, which are distributed to the integument and mucous membranes 

 of the face, are exclusively sensitive ; while the branch of the same 

 nerve distributed to the muscles of mastication consists principally or 

 entirely of motor fibres. 



No essential distinction is perceptible, in anatomical characters, be- 

 tween sensitive and motor nerve fibres. In nerves which perform a 

 motor function, the fibres are for the most part of comparatively large 

 size, averaging 15 mmm. in diameter; while in those performing a 

 sensitive function they are smaller, averaging not more than 10 mmm. 

 in diameter, and many of them being considerably less. But this is 

 only a difference of numerical proportion between the larger and smaller 

 fibres ; since both large and small fibres are found in both motor and 

 sensitive nerves. Even the motor fibres become reduced to the smaller 

 size before terminating in the muscular tissue ; and the nerve fibres 

 generally are diminished or increased in diameter on passing into or 

 out of the gray substance of the nervous centres. No absolute dis- 

 tinction therefore can be made between sensitive and motor fibres as 



