CHAP, ix.] NERVOUS SYSTEM AND ORGANS OF SENSE. 307 



ception of FORCE itself, of which so much has been made by popular 

 teachers of our day. Force, indeed, has been, and is, constantly 

 spoken of as if it were a substance ; as if indeed it were the only 

 substance. But to plain minds, as well as to followers of the highest 

 philosophy that of Aristotle that which exists, as manifested to 

 our senses, is the external world of visible, audible, tangible, sapid 

 or odorous substances, which substances indeed possess many active 

 powers. Force is in reality but an abstraction ; it does not exist 

 either for our senses or our reason apart from substances, and is the 

 name applied to the activities of such substances considered abstractedly 

 from the acting substances themselves. A living body is a special 

 substantial whole made up of parts, and both the whole and its parts 

 have various active powers, and the active powers of animals, 

 energizing through the nervous system are really what is meant by 

 the abstract term " nervous vital force." 



29. We may now pass to the consideration of the functions of 

 the different parts of the nervous system. Before doing so, however, 

 it should be observed that certain CONDITIONS are NECESSARY for the 

 continued exercise of all nervous activity. 



Thus the temperature of the body must be moderate, certainly not 

 less than about 72, or more than about 120. The nervous tissue 

 must also be adequately supplied with blood. This blood must be 

 sufficiently oxygenated, and also devoid of poisonous matter, such, e.g., 

 as that with which it becomes charged from a cessation of the renal 

 secretion. Finally, the continuity of the more important nervous 

 structures must be maintained. 



30. The FUNCTIONS OF THE SPINAL NERVES are manifestly 

 both sensory and motor, according to their distributions and con- 

 nexions. If one of these nerves be divided and the cut end of its 

 distal part be irritated, motion ensues in the muscles to which such 

 nerve is distributed, but no pain ensues from such irritation. If, on 

 the other hand, the cut end of its proximal part be irritated, pain is 

 caused, but not motion. If the posterior root of a spinal nerve be 

 alone severed, the parts supplied with twigs from such nerve only 

 lose their sensibility, but their power of motion remains. If, on the 

 other hand, the anterior root of a spinal nerve be alone divided, then 

 the parts supplied by such nerve are paralyzed as to motion, but 

 nevertheless retain their sensibility. 



It has therefore been concluded that all the nerves conveying 

 influence inwards, and centrally (called afferent nerves, and giving 

 rise to sensation), pass through the posterior roots of the spinal 

 nerves exclusively, and that the fibres which convey motor 

 influence outwards and peripherally (called efferent nerves), pass 

 through the anterior roots of the spinal nerves exclusively the two 

 sets being mingled at and beyond the point of junction between the 

 roots, but sensory and motor fibres being distributed in the ramifica- 

 tions of each spinal nerve. It has been further assumed that the 

 nerves themselves neither feel nor initiate motion, but that both 

 feelings and motor impulses arise in the grey matter of the nervous 



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