268 A Manual of Veterinary Physiology. 



Between the spinal cord and the sympathetic system are 

 chains of fibres connecting the two ; these fibres originate 

 in certain columns of the cord, pass out through the inter- 

 vertebral foramen, and so establish a communication 

 between these two important nervous systems (see p. 261). 



In order that we may understand the functions of the 

 nerves and spinal cord, it is necessary that the complex 

 acts which they are capable of producing should now be 

 dealt with. 



Nerves by themselves are not capable of generating any 

 impulses ; they must be in connection with a nerve centre. 

 We have seen that the spinal nerves are in connection with 

 nerve centres in the spinal cord and brain, and we have 

 now to learn the nature of the various impulses these are 

 capable of causing. 



Reflex Action. — When a horse endeavours to save himself 

 from falling the act is a purely reflex one — that is to say, 

 whether he voluntarily tries to save himself or no the result 

 is the same, viz., the effort is made ; he becomes conscious 

 of the effort, but only after it is completed. When the 

 eyeball is touched with the finger it is withdrawn into its 

 socket, the membrana protruded, and the eyelids closed ; 

 this is a purely reflex act, and may be found to occur 

 immediately after death produced by section of the medulla ; 

 with the exception that the eyelids are not closed, the act 

 in all other respects is as perfectly performed as during life. 

 If the hand be suddenly raised as if to strike a blow in the 

 face, the eyelids blink, and by no effort of the will can this 

 under ordinary circumstances be prevented, though the 

 horse may know through repeated attempts that no blow 

 will be inflicted; the rapid closing of the lids is a reflex 

 act. 



The main feature of a reflex act is its apparently inten- 

 tional character. By means of the sensory nerves the im- 

 pression is conveyed to a nerve centre, and in this place 

 certain changes rapidly occur which load to impulses 

 passing out along the motor nerves. The changes in the 

 norvo centre are probably of a highly complex kind. The 



