CHAPTER XXVIII. 

 GENERAL PHYSIOLOGY OF THE NEKVOUS SYSTEM. 



ANATOMICAL SKETCH. 



THE nervous system is the apparatus by which the distant parts 

 of the body are kept in constant relationship with one another, 

 so that a change of state of any one organ is communicated to 

 and may set up corresponding changes in remote parts of the sys- 

 tem. It is made up of two varieties of tissue, both of which pos- 

 sess special vital properties. The one, which is composed of thread- 

 like strands of protoplasm nerve fibres connect together the 

 elements of the other group, the nerve corpuscles, which form 

 either peripheral or central terminals. Nerve fibres are then 

 simply special conducting agents, having at one extremity a spe- 

 cial terminal or nerve cell for sending impulses, and at the other 

 extremity other cells for receiving the same. These terminal 

 organs, between which the nerve fibres pass, are the agents which 

 determine the direction the impulse is to travel along the nerve. 

 The sending organ is sometimes at the peripheral end of the nerve, 

 and the receiver in the nerve centres, as in the case of an ordinary 

 cutaneous nerve, which carries impulses from the skin to the 

 brain ; or these duties of the terminal organs may be reversed, 

 as in the case of the nerves conveying impulses from the brain 

 to the muscles. 



The former kind of nerves are called afferent or centripetal, and 

 the latter efferent or centrifugal. Nerves are capable of carrying 

 impulses in either direction, as has been proved by cutting the 

 afferent lingual and the efferent hypoglossal nerves, and causing 

 the proximal end of the former to unite with the distal end of the 

 latter, which is distributed to the muscles of the tongue. When 

 the union has taken place a stimulus applied on the efferent por- 

 tion causes the muscles to move. 



