NERVOUS SYSTEM. 



75 



me 



It is also through the medium of the nerves that the influence 

 of the WILL is communicated from the brain to different parts of 

 the body, and that motion is performed. 



22. Indeed, if we cut the nerves which go to a limb, it becomes 

 immediately insensible, and ceases to execute voluntary motion, 

 or in other words, it is paralysed. 



23. Certain nerves serve only for the Fig. 26. 

 transmission of sensations, others serve 



only for motion, but the greater part fulfil 

 both these functions at the same time ; this 

 arises from the union of a certain number 

 of nervous fibres, of which some possess 

 the first of these faculties, and others the 

 second. At the point where the nerves 

 issue from the spinal marrow, these two 

 species of fibres are still separate, and con- 

 stitute two distinct roots, one situated be- 

 fore the other (/%. 26.) ; the anterior root 

 serves for motion, and the posterior for sensibility. When in a 

 living animal we cut the anterior roots of all these nerves, it is 

 incapable of moving, but preserves its sensibility ; while if we 

 cut the posterior roots without wounding the anterior, the con- 

 trary is true. 



[The gang Home nervous system^also called the great sympathetic 

 nerve, or nervous system of organic fife, is composed of a number 

 of small, very distinct nervous masses, which are united to each 

 other by medullary cords and different nerves, which anastemose 

 (communicate by branches) with the cerebro-spinal system, or are 

 distributed to the neighbouring organs. These nervous centres 

 bear the name of ganglions: they are found in the head, neck, 

 thorax and abdomen. Most of them are placed symmetrically on 

 each side of a middle line, in front of the vertebral column, and 

 thus form a double chain from the head to the pelvis; but they are 

 found in other parts : near the heart, for example, and in the 

 vicinity of the stomach.] 



Ex pin n at. of Fig. 26. me A portion of the spinal marrow, showing tho 

 manner in which the n< rves ari.se by two sets or bundles of roots. r. tmterior 

 roots serving for motion. rp. posterior roots serving for sensibility. g 

 ganglionic swelling of a posterior root. 



2*2. What effect would be produced by cu? ting the r/frves which goto i limb? 



'J3. Do all nerves perform the same functions? How is it that some 

 nerves serve for motion and also to transmit sensation ? Which nen'ous 

 roots serve for motion ? Which nervous roots serve for sensation ? If we cut 

 the anterior roots of the nerves in a living animal what happens ? What r 

 suits from cutting the posterior roots ? 



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